aten Design Group has a great article on why they chose Drupal. I would be hard pressed to tell you the number of high quality shops who have made the same choice. Many of us have set about building a better content management system. One better suited to our particular needs or our view of how to do content management. As someone who has used content management systems dating back to Clay Basket and NetObjects Fusion as well as rolling my own, Drupal simply makes it possible to focus on doing the things we want with a content management system (and yes it's more than just a CMS) instead of spending weeks and years building one.