From Bayosphere (http://bayosphere.com/why-drupal):

Grassroots Journalism, Creative Commons, and Open Source Software share visions of the free flow of ideas. There are countless Open Source content management systems, several of which could have served as an excellent starting point for Bayosphere, but we chose Drupal for its combination of flexibility, community and maturity.

Drupal has a stellar philosophy of extensibility. On an old school CMS, changing the system's behavior required changing the software package's source code. Eventually the package requires an upstream upgrade (for security fixes, if nothing else), and a programmer has to reapply every local custom change ever made, an often-tedious and sometimes-impossible task.