As a developer, why the heck would I want to put BG&E 2 (or GTA IV for that matter) on Wii? That doesn't make any sense to me, even with the massive Wii install base. With both BG&E 2 and GTA IV, the goal from a dev standpoint would be to:
1) Take the game world to the next level, making it larger and more immersive
2) Do things that couldn't be done on last-generation hardware
This eliminates Wii right off the bat. Ancel's already done BG&E on last-gen hardware, and Wii is essentially last-gen hardware. What more could he do with a Wii version that hasn't already been done in the first game, aside from motion control? It would be a bit better graphically, but it would be impossible for it to be a quantum leap over BG&E like, say, GTA IV is over the PS2 GTAs.
For me, BG&E 2 being PS3/360 only is a no-brainer. If developers want to release an upgraded version of an IP already released on GCN/Xbox/PS2, it isn't going to be on Wii because its hardware isn't appreciably different from GameCube. It's simply not a "next-gen" console in that respect.