I think they cant come up with a compelling story. plus, now that all nobodies are gone, that removes most of the "grown up" enemies. Also, I beleive they are trying to make the game a character choice game where you can play as riku, sora, or kairi.
I have no information to back thaqt up, I just feel it. How awesome would that be. also, they need to add axel and aqua to dissidia