I often get asked this question.... so here is my answer....

Most have been pulled from other sites, and links to those sites have been included... The last section are my own personal thoughts...

Here are some ‘points’ with explanations I’ve pulled from, http://www.volacci.com/blog/ben-finklea/2010/march/31/9-reasons-why-drupal-best-cms-seo, which starts with:

If you are in need of a search engine-friendly website built with an open source content management system (CMS), Drupal is your knight in shining armour.
Drupal is a great platform for building your website. It is a very powerful system that can be used for large, complex sites like www.whitehouse.gov, www.mensa.org, and http://www.lefigaro.fr, the oldest and second-largest newspaper in France. Some other sites you may also have heard of include: The Discovery Channel, Warner Bros. Records, The United Nations, and General Motors.
It’s widely believed that Drupal is one of the best choices if you want to rank well in the search engines. I personally believe that it’s hands-down the best possible platform for SEO. I’ve seen clients triple their traffic within a few weeks of switching from a lesser platform.
It then lists:

Search engine optimized URLs. Unlike WordPress, Drupal gives you complete control over URL structure. Each item of content created in Drupal (called a node) can be given a custom URL, called a URL alias. You can even set it to automatically generate user- and search engine-friendly URLs based on content type (blog post, page, user), title of the post, keyword, date, time, or any other information related to the node. In WordPress, you are generally limited to one type of permalink URL for all posts. You can override it, but it's much less specific than Drupal's URL aliases. For years, Joomla was lacking in search engine-friendly URLs and the ability to fix them. It now offers a built-in "search engine-friendly" functionality, but it lacks the functionality and control over the URL's parameters.

Ease of editing and revisions. Even the most professional companies make mistakes. Drupal's built-in version control allows you to save new versions of your Web pages every time they are edited. This gives you the ability to revert back to old versions at any time. Want to try a new marketing message on your site? Type it in. Didn't work? Revert to the previous page.

The organizational wizardry of taxonomy. Drupal has a very powerful, built-in taxonomy (categorization) system that allows you to organize and tag content with keyword-rich terms. For example, you can use free tagging for types of content like blogs or products, while your news or tutorial sections might have a list of categories that can only be selected from a dropdown that is defined by the webmaster. You can also have hierarchical categories, with single or multiple parent categories.

Multiple user management. Drupal is almost as well-known for its community as it is for being easily optimized. That's because Drupal was designed for community-based websites, and has a strong user role and access control functionality. There are no limits to the user roles and access levels you can create and customize - for example, "anonymous visitor," "authenticated user," "editor," "webmaster," "admin," and "moderator."

Page titles. The page title is the single most important on-page thing you can do to improve the SEO of your website. Page titles, the line of text in the HTML of a Web page that summarizes what that page is about, act as the welcome mat for your website. Drupal can generate a page title automatically by using the site name and node title (product title, news article title, etc) for individual nodes. You can also take control by writing your own page titles anywhere you can create content.

Better integration with Google Analytics. Have you ever wondered how much your own clicking around and working on your website skews your analytics data? Not if you have a properly configured Drupal site. Using the Google Analytics module for Drupal, you can dynamically show and hide analytics code based on several factors, including if the user is logged in as an admin. You can even not track certain sections of your site, not track certain user types (like people logged in as company employees), and cache the Google Analytics code locally, which will speed up your page load times. All within Drupal. No programming required.

Passionate and active community. Drupal has a large community of users and developers who are very active and passionate about the CMS. More than 650,000 user accounts have been created on Drupal.org, and more than 2,000 people have registered developer accounts. You can find hundreds of community-contributed modules that help make Drupal a better experience. Drupal forums offer support and you can find other Drupal users in Drupal Groups.

Drupal SEO is easy. Just use the Drupal SEO checklist module, go through the simple, step-by-step instructions, and you're well on your way to the best SEO experience you've ever had on a CMS. (https://drupal.org/project/seo_checklist)

Stable! It is a powerful and flexible framework for creating dynamic, reliable, and scalable websites. Drupal uses a database abstraction layer with support for several engines such as MySQL and PostgreSQL. Along with support for database table prefixes, Drupal has been extensively tested under high loads (such as a "Slashdot effect") and also includes built-in throttling and caching for optimal performance.

…and some more from here: http://www.ross.ws/content/why-drupal-best-choice-building-our-website

Drupal is a powerful and flexible framework for creating dynamic, reliable, and scalable websites. It has built-in support for custom content types, rich editing, and content revisioning; user comments and forums; clean and search-engine friendly URLs; modular theming; tagging with keywords and taxonomy terms; image galleries; support for an unlimited number of web pages; user authentication, sessions, permissions, and management; internationalization and localization; and much more.

Site content and settings are saved for you in a database, which can be backed up (manually or automatically), for greater security.

Drupal allows website owners to add and modify the content easily, without having to understand HTML or other technologies.

Drupal is optimal for building search-engine friendly websites, including standards-compliant HTML/CSS, dynamic page titles, meta tags, customizable and readable URLs, RDF support, and Google Analytics integration.

It is based upon the most commonly used and trusted Web technologies available, including PHP and MySQL.

Drupal is actively maintained, tested, and improved — including updates for any security problems discovered.

Drupal is more secure than the only two CMSs more commonly used, WordPress and Joomla. Drupal's dedicated security team has more than 40 experts, who will unpublish any modules with unresolved security issues. Also, Drupal's password security meets the requirements for US government agencies. In fact, the White House website was built using Drupal.

Drupal is built using high-quality, modular code that has been well tested, during its successful 10-year history.

It offers high performance, with built-in caching and scalability to multiple servers.

Almost half a million websites worldwide run on Drupal, including those for such demanding clients as Fortune 500 companies, universities, non-profit organizations, and governments. Wikipedia, Sony, Viacom, Warner Brothers, Yahoo, and others run high-traffic Drupal websites, and have made strategic business decisions to invest in it.

Here is a nice bit of text that pretty much answers the question in one go… it’s here for reference: http://www.avenuewebmedia.com/why-use-drupal and goes like:

We have chosen Drupal as our development platform of choice after comparing it to just about every other platform and CMS available.

Drupal is an Open Source Content Management System (CMS), designed for power and flexibility. Considered by many as the most important platform for the PHP programming language, Drupal has a large fan-base of professional developers that use the software for all sorts of commercial projects, and often contribute to extend Drupal with their own work.

Drupal excels as a CMS and also as a development platform. It has a strong core capable of large flexibility. This means that the system can support many types of features with ease, from shopping carts to user profiles, calendars, galleries and beyond. Drupal can handle just about anything you can imagine. As developers, we embrace this flexibility because it allows us to turn any website dream into reality.

Drupal is also robust and safe. With over 700 developers working on the latest version of the core, a platform like Drupal has endured rigorous testing and deep scrutiny before it is allowed to become part of your website.

Drupal is also very cost effective because it allows us to develop complex sites in a fraction of the time it would take using other technologies. With thousands of community-contributed modules extending the flexibility of Drupal, chances are that Drupal already supports whatever feature you can imagine; and even if we can't find a module that suits your needs, we can build new modules relatively quickly.

Also, and perhaps just as important as everything else, Drupal can be setup to be extremely user and search-engine friendly, making your site a pleasure to navigate and a hotspot for the search engines.

And of course, Drupal gives you direct access to manage your site. We can set you up with various levels of permissions and access to different areas for registered members, contributors and content editors. You can update the site yourself and/or delegate content management tasks to other members of your team. Content Management with Drupal can be as easy as typing a letter on Microsoft Word, and you have instant access to each and any area of your site.

For all these reasons Drupal is the clear choice, and the choice of many other professional development companies as well.

… and my opinion… in my own words…

I think you really need to look at things from a user point of view… Yes, Drupal was designed and built from a different perspective to other CMS systems like Wordpress, so it is therefore more powerful and equipped to deal with larger, heavier tasks… But it was also designed to be ‘Open Source’ so you don’t have to pay for an expensive license, for a system that can’t be easily fixed, improved or expanded, as in some enterprise solutions… But put all of that technical jargon to one side for a moment and look at the system from an administrators point of view, someone sat in the office having to add or edit something…

In my opinion, it’s the way Drupal handles ‘content’ that crowns it ‘king’… Each news article, event, product, office address is entered into the system as a piece of ‘content’… These pieces of ‘content’ are organised into groups called ‘content types’. Each ‘content type’ can be set up with its own unique fields, dropdowns, image uploads depending on what is needed. So a site ‘administrator’ needing to create a new ‘news article’ can simply click ‘add new news article’, fill in the fields of the form, and the system does the rest… This makes adding or editing really easy for the admin team.

Drupal allows staff to be given an access level in the form of a ‘role’, i.e. ‘administrator’, ‘blog author’, etc. Access to add/edit a piece of content can be restricted to ‘roles’, which means that certain members of staff can be restricted to only adding or editing content they’re responsible for… i.e. we could create a role called ‘recruitment’ and create user accounts for the careers team allowing them to only add/edit ‘Job Vacancies’ on the site. Another role could be ‘news author’ which again allowed a certain group of people access all of the news articles, but without allowing access to ‘Job Vacancies’, or other ‘content types’. Meanwhile and ‘administrator’ has full access to add/edit all ‘content types’ as well as some of the system settings.

This allows a company website to become very active with up-to-date content from a variety of people without anyone treading on anyone else’s toes, yet allowing the admin team full control over everything. In short, ‘Professional power and flexibility in a real world scenario’….