Using Mario Kart is a poor comparison. Mario Kart is basically the normal entry for a Nintendo system (console or handheld), and granted will sell units because it's fricking Mario Kart. (not to mention they are quality entries, except MK: Super Circuit).
NSMBWii is the first 2-D Mario console game in almost 20 years. A typical game entry this is not. This is a big deal, and obviously, will sell A LOT of copies. (and to test the Wii Demo Play, which is completely optional so it won't destroy the game k guys hur hur)