It was successful because it was cheap, Nintendo marketing, hype from Nintendo loyalists, the potential that hardcore gamers found in motion controls and the accessibility that non-gamers found with them.
And later, the same things in relation to Wii Fit.
Bet with Dr.A.Peter.Nintendo that Super Mario Galaxy 2 won't sell 15 million copies up to six months after it's release, the winner will get Avatar control for a week and signature control for a month.