FFXIII should not have gone to Wii, but other games should have. Who would want to play FFXIII on the wii over the 360 or PS3? I know I wouldn't. Might as well do a remake of FFVII if you want to sell something on the WII. Or some other RPG. Each new FF game is made to push the graphic capabilities and the emersiveness of the genre. If it was on the WII it would be counter productive to what making a new FF game is all about.
Edit to respond to your Edit: Because Square Enix is a JRPG maker. And uhm well the xbox 360 doesn't have much of a JRPG market and the PS3 does. It's pretty obvious.







