Wordpress, Joomla & Drupal are the most popular Content Management Systems (or CMS) out there.
They are extremely flexible, and in some cases allow you to program an entire web platform
without knowing how to code (which is sometimes a philosophy I deeply disagree with).

It is interesting to note though, that although extremely popular and very well written,
those systems do not use the popular PHP frameworks that a great majority of programmers use when starting a PHP project from scratch.

Codeigniter, Yii, Zend, Symfony, CakePHP, are probably the most popular PHP frameworks
out there (Yii being my personal favorite), and they are very robust and well built (not to mention well documented).
Why should a programmer, when trying to write a Module or even understand the Core,
re-learn principals of a completely new systems just because the popular CMS's
refuse to use the most powerful frameworks out there?

But fear not, seems that Drupal is the first to give hope on the subject.
Drupal 8 is going to be using a lot of the major components that Symfony 2 is using.
Having relatively little experience with Symfony, I would have to look into it to give my opinion,
but it is definitely a step in the right direction