They did use the proprietary VBulletin software for their forum.
The reason probably being the way that Ubuntu works in general nowadays: 'good' quality proprietary software over free software. They saw that VBulletin was a 'good' solution, didn't care whether it was free software or not, and used it.
To be honest this isn't surprising to me since Ubuntu hosts quite a lot of proprietary services now (Ubuntu One, their forum, etc). As for why they didn't just write a patch for an existing free program, just think about it: why haven't they created a patch for something like the free Nouveau driver or Radeon driver to improve it instead of making the decision to include the proprietary drivers in their distribution?
Ubuntu nowadays just doesn't care about free software. Hell, isn't that the reason Trisquel exists?
Anyway, there were plenty of forum choices for them to use, the most well-known being phpBB or even Simple Machines (SMF) v2.0+ (released under the BSD license).
Also, I'm pretty sure that the German Ubuntu forum isn't run by Canonical.
