I'm sure MP had a yearly release for a while because, well, they had a developer to do them and then needed to fill out their release schedule.
Point is, Nintendo has been doing some things differently with the Wii ever since Iwata has had full control of whats going on, and yearly sequels to games that could become ever green if given the right support is not gonna happen anymore. Who wants to buy essentially the same game over and over again every year besides the sports fanatics that need an updated roster?
If it's really just because Hudson decided to not make them anymore, then so be it, but that doesn't change the fact that there is only 1 MP game for Wii and Nintendo is letting the game ride on it's own merits and it's doing just fine. We have yet to even hear request for a sequel or interest in them making one.