Because Sora isn't owned by Squeenix, he's owned by Disney. Disney's probably not gonna let their property be in the game with the mass murdering psycho god killer. Disney is a family friendly company that let's only family friendly things go near their stuff.
Why do you think the only Sony title they've touched is LPB? What about the Kinect, which they've made several exclusive games for. They only like family friendly stuff, if they saw Kratos they would freak out and decline. Disney also is a huge company, which means more money to pay. Asking for characters isn't as simple as "yes" or "no". It requires contracts and tons of legal buisness. Disney has also said that they will not let use of Sora in any non-Disney game.
There's also the fact that the Kingdom Hearts series is a tangled up mess of terrible plot elements and characters.
So go ahead and leave your stupid "But Sora NEEEDS to be in!" one line posts, I don't **bleep** care.