Actually, one Mario platformer a gen has kind of been an aberation. That wasn't how it was with Nintendo's really successful consoles the NES, SNES, original Gameboy. They all had more than one original Mario platformer, so it doesn't surprise me that the Wii is going to as well.
However, each of the Mario platformers can have new storylines to hinge their gameplay upon while I think a kart racer is more limited in that respect.
I think Nintendo could put another Mario Kart game on Wii but I wouldn't expect it until later in the gen when the first one's sales have either cooled off or they can make some significant improvement over the first which might not be possible until their next console.
One possible reason they could be gearing up to release two new Mario platformers though could be because they think that people that enjoyed Mario Kart will also be willing to pick up the new Mario platformers. And Mario Kart Wii's success could be a big part of why New Super Mario 2 features competitive multiplayer gameplay and the ability to skip difficult parts.







