Well, both critics and players liked it. It's one of the highest rated titles in recent history. It's a core title, but casuals like it a lot too. Basically the game appeals to almost everyone, whether they've played games for ages or not. For games like that, word of mouth really goes a long way. Also, it's Mario. Any Mario title sells tons.
And yes, the advertising is sure to have helped this game, too. I remember seeing tv-commercials for Galaxy just a few short weeks ago, while it released like 2 years ago. That's not something you see a lot. Well, apart from other evergreen Nintendo titles (I still regularly see commercials for Wii Fit, Brawl, and Mario Kart too :p).
Nintendo Network ID: Cheebee 3DS Code: 2320 - 6113 - 9046