There are many possible reasons. Perhaps they feel their events are more likely to get readers there. Maybe they have an account there and are not inclined to sign up for yet another forum. They might even fear the heavy hand of censorship (not, admittedly, likely for a discussion about a brony panel . . . but in general, it is rarely a good idea to allow an event full control over discussion of its activities).
You could just as easily ask why ConFuzzled set up its own forum system rather than request expansion of the UKfur forum dedicated to it - there are good arguments both for and against doing so.