it's easy to find and hire people who are familiar with it.
who *think they* are familiar with it.
Drupal looks easy to extend for a beginner developper because, like php, you don't have to do something right to have it work.
So it's really quick to have a mess of a codebase that leads to an unstable site that's a nightmare to maintain.
But if you really know the insides of it, you can craft a something beautiful that runs smoothly and is easy to work with. It's not perfect, and "real" coders will look at its non-use of OOP with contempt, but it's pretty good at what it can do.
And, let's be honest for a second: it's one of the least horrible open source CMSs out there (at least in the php world, I don't really know about Java or python or other CMSs).
If you want a good pphp cms, you have Drupal or Ezpublish. There's wordpress that wants to be a cms but isn't quite there, Joomla is a joke, Typo3 is from another time... The perfect software doesn't exist.