I think the reason why some games aren't being developed on wii is due to the fact of the surprise of the wii's popularity. Alot of people weren't expecting the wii to be near the success it is, so they began developing on the PS3 or 360. Now, a lot of developers are jumping over to the wii because of it's success.
The reason why we aren't seeing ports come from the PS3 or 360 is because a lot of engines that run those games can't be run on the wii. So, that's why the wii is getting a lot of PS2/PSP ports early on its life. Right now though, 2008 could seriously be the year for wii. (And, I'll say PS3)