@Alpha, I see your point, but the difference between your comparisons with Super Mario Galaxy and Metroid Prime 3 is that Mario Kart appeals more to a casual fan base then either of those two games do. Racing a go-kart is something that even casual fans will find fun and engaging especially with the new Wii steering wheel controls. This means that game will be able to target all three major groups of gamers: hardcore, mainstream, and casual. This fact equals more sales.
2