Brand loyalty doesn't matter to corporations....
Besides, Kingdom Hearts series is not a Sony exclusive: Kingdom Hearts Chain of Memories was released for the Gameboy Advance, 1 year before Kingdom Hearts II.
And there's a Kingdom Hearts game in development for the Nintendo DS.
There's is a good chance that Kingdom Hearts III would be a Wii exclusive... just look what happened to Monster Hunter 3, it was announced as a PS3 exclusive, and now is a Wii exclusive... same with Fatal Frame.









