People tend to forget the Wii line up:
Driver: Parellel Lines,
No More Heroes,
Man Hunt 2,
These 5 games are all up the same alley as GTA, and Man hUnt even from Rockstar, so in my opinion it comes down to sales. If they see titles like the Godfather which is getting descent reviews sell well, and their very own Man Hunt does well, I don't see why they wouldn't explore their option on the Wii for a GTA title. And like it was already said, they could release a new GTA title for the Wii/ps2 and possibly the psp also.
And for the comment about Wii controls not enhancing the gameplay of GTA, just check out a video of Godfather gameplay.