I'm fine with this game except why did he/his company have to make this a Vita exclusive?
Because he wanted the game to be a Vita exclusive. Believe it or not, but 3rd party games
can occasionally still be exclusive. You don't see me bitching and moaning that Radiant Historia and Ghost Trick haven't been announced for the PSP or Vita because
it's so unfair for the DS to have these games for themselves. Though it is.
Why not port it to the 3DS or some other system also and therefore potentially make more money?
Because putting a Pikmin-esque game on a Nintendo platform that could potentially have its
own Pikmin game from Nintendo in the near future is a total waste of resources. If a new Pikmin gets announced for 3DS, that'll kill any sales for this game just as Nintendo tends to drain all the money out of the market for 3rd parties whenever they release a big franchise game.
Is Sony paying him off for this to be exclusive to their system?
That's not out of the realm of possibility. Sony's been known to do that in the past, and they really want Vita to succeed. We'll see. Keep in mind that this game probably started production earlier this year, when the 3DS was tanking and companies were delaying games just to
not have them on store shelves when no one would buy them. That probably had a lot to do with going with Vita.
Or is he just doing it because he had a falling out with Nintendo and wants to stab them in the back?
I highly doubt that this is as personal as it seems you want it to be. He has experience working on a Pikmin game, and he's making a game in a style he's familiar with. That's probably all there is to that.