Why not CakePHP or Zend Framework or Limonade or Symfony or Solar or Kohana or Zoop or Yii or Akelos or PHP on Trax or Prado or Seagull?
Because you've gotta pick one, dammit
All of them have value
That being said, CI is… Easy to understand Simple doesn't require advanced OOP Doesn't force lots of conventions Plays well with others Quick to get up and running Good docs and great community Backed by invested entity (http://ellislab.com)