I think a lot of it has to do with immediate competition from other Nintendo products. Namely Mario Kart Wii, Super Smash Bros Brawl, and Wii Fit. I think if the game had a longer slot of being out without those games releasing (at least a good 6 months) then it could have sold better than it did. I think the whole 2d vs 3d is most prominent in Japan but in America and Europe the differences probably aren't as big as we make it out to be.
Thus looking forward to Super Mario Galaxy 2 I think this game can open pretty high and as long as there isn't another big Mario game or casual game (Zelda won't cause a drop in sales of it) in the immediate future it should be able to sell well over a longer period of time.
But hey Galaxy is still selling even now and should end its lifecycle with some very respectable sales.
Essay Pro