Why not? I'm not intending to be malicious or smart-alecky here.

I've been called for two different cases of compromised Joomla installations. In one case, the 1.0 to 1.5 upgrade process was completely broken; I couldn't get it to work without hacking up the upgrade script.

I didn't play around with it long enough to remember anything about its file or database architecture. But, I think if I were going to develop a CMS, I would want to make it easy to patch and upgrade, and I would want to make sure that it was somewhat secure.

Joomla isn't nearly as extensible as Drupal. If you're just trying to put up a few pages and call it a day, it's great, but Drupal can handle practically everything. I use it to mock up web sites because I can't program: It's possible to use Drupal to create a functioning beta of a fairly complex site without writing a line of code. It's slow and results in lots of bugs, but it works.

For building sites or for building apps? While Drupal can be used to build a great web site for a company (and often is!) Joomla's extensibility seems aimed more at rapid application development using a strong mvc framework.

If you don't mind relying on 3rd party contrib modules and external development roadmaps, you can build apps in Drupal, but if you just want a framework to build and deploy your own web apps Joomla is fully extensible in the OOP sense.

Also Joomla is classier than Drupal. :-P