Almost every week or so, someone asks me:
Why PHP? Apparently, you are doing Java too. So why not Java? Do you regret the fact that you wrote Drupal in PHP?
No, I don't regret the choice of PHP. Both languages will get the job done, but Drupal's main target audience are not conservative verticals (government, healthcare, banking).
The web is built by millions of individuals, many of which are amateurs. They continuously update, tweak and rebuild their websites. Scripting languages like PHP lend themselves to that, and are widely available at affordable cost. Sun, on the other hand, failed to make Java accessible to amateurs.
It would have been very difficult to get critical mass if Drupal was written in Java.