There are millions of good reasons why this game has sold well, I don't see how it's that surprising.
Very simply, 2D Mario games have always sold well and there hasn't been a home console 2D Mario game for a very long time. The Wii is currently very popular, has a very large userbase who certainly like Nintendo first party games, it went on sale just before Christmas, it has local multiplayer and although it didn't get 99% on Metacritic it's still an awesomely fun game that is going to benefit from positive word of mouth.