I hate to say it, but the argument "the Wii has a much higher install base for units" argument doesn't really work.
Merely because SMG is a very Nintendo-Oriented game. It won't sell to the droves that are typically buying the Wii (they buy it for the more casual fare).
Zelda: Twilight Princess has done "mediocre" considering the absolutely insane install base. If Zelda: TP can only manage ~4m units on the Wii, why should SMG sell 2.5x or more, higher?
Back from the dead, I'm afraid.