Why use Drupal over Wordpress?

• Wordpress was designed only to be a blog with some easy add-ons.
• Drupal was designed to be more of a generalist: it’s for making ‘anything’ and is far more robust.
• Wordpress could be the better choice for blogs since it is better at being a blog than Drupal.  This is something of debate.
• Wordpress is still a sound choice of CMS for SEO and security; so if wordpress satisfies a simpler project’s requirements then by all means use it- it is easier and faster to set up than Drupal.
• Wordpress is not designed to be highly scalable to many simultaneous users, nor does it have flexible roles, permissions, extensible content types, nor does it have plentiful well-tested, quality add-ons.  It has a few and a lot of really poor plugins.
• Caveat: Trying to force Wordpress to do something it cannot do easily with very popular plug-ins can be worse than suffering the learning curve of Drupal.
• It has superior session handling for a CMS.
• It has superior security.
• It is a more consistent, reliable and flexible framework for development.
• It is considered better for SEO from our research.
• It uses a ‘separation of concerns’ architecture to cleanly and consistently separate structure, function, form, and presentation in layers (ie: php from data as db/xml, layout and presentation as html and css).  
• It heavily uses ‘defaults overrides’ in code in the form of hooks and in themes in the form of templates.  This makes it extremely flexible.
• Other CMS’es do a very very bad job of at least one of the above.
• Drupal has a steeper learning curve than wordpress or Joomla.
• Drupal and it’s developers make no excuse for this fact- it is a robust, flexible tool
• That said, the drupal community is constantly addressing usability and user-experience issues because they want the industry market share.