They make sequels to games they can improve and do new things with. They significantly improved Wii Fit Plus over the original, and Galaxy 2 they already stated that they felt they hadn't fully explored the galaxy theme (i don't have a link sorry).
Mario Kart Wii has already explored everything you can do with it other than brand new courses (which would rock) but doesn't really add to the gameplay or bring it to owners of a new console. Honestly, I'm happy with how MKWii is and would much rather them spend the development time elsewhere.








