Dan Houser, co-founder of Rockstar Games, reveals in a interview why they didn't stick with the Wii:

Dan Houser: It didn?t feel natural to us, I guess. It really was that the DS felt like it had a lot of interesting challenges that would be totally different from what we?d done in the past. The stylus and the chance to use minigames in that way was really interesting and exciting to us, and we thought we could integrate seamlessly between those two modes. And it would be the chance to make something really good on a handheld with our handheld-focused team. That was really why we went that way. We haven?t really done any concrete, major thinking about the Wii, one way or another. They?re sort of separate issues.

Thanks to GoNintendo for this bit.