To name a few things, translation support, integration with the ubuntu bug tracker, PPA management, blueprinting system, and much more powerful bug triaging.
Yo aroman, moving to GitHub has been discussed several times in the last months, we even have a couple of projects there (www.github.com/elementary), but besides all the reasons you named it's also proprietary software. Still, I believe the biggest reason is translations and PPAs.
...which means we can't set up our own instance of it or modify it work as we need (as I outlined above, in its current form it only does a fraction of what we would need it to).
Please don't superimpose your own views about software purity on our practical realities.
To name a few things, translation support, integration with the ubuntu bug tracker, PPA management, blueprinting system, and much more powerful bug triaging.