Better question is, why doesn't bitcointalk upgrade to xenForo? Scores better than anything else out right now... it doesn't look like it came from the 90's!
Just looked over xenForo and it really is a nice piece of software. I think it's time BT take it up a notch and start using what has already proven itself over the years instead of trying to develop a new piece of software. I mean it's 2012 and this forum uses a thread to document all good trades. Just my opinion though.