I'm actually going to have to go with "it's a Nintendo game".
Look at the best selling games on Wii;
Wii Sports and play can't really be compared
Wii Fit and Mario Kart Wii have far better legs. Wii Sports Resort will likely fit in here.
Mario & Sonic @ the Olympics has somewhat better legs. As does Mario Party 8.
Of Nintendo's top 10, only 2 games are not outlegging Super Mario Galaxy. Super Smash Bros. Brawl, which practically ties SMG, and Twilight Princess, which is a Zelda title - Nintendo's perhaps worst legged multi-million selling franchise.
But again, that actually fits with your poll, because why do Nintendo's titles have such good legs? Definitely word of mouth.