Vast majority : Marketing towards casual gamers & innovation/novelty of motion control. To a lesser extent, being the cheapest console (at the time) helped as well as a free game (Wii Sports)
minority : Nintendo fans (a Zelda game and Mario Galaxy to come).
I personally don't think Natal or Move will do well, because the novelty of motion control is now 4 years old; and Ninty have cornered the casual gamer market. I can't see casual gamers buying an Xbox or PS3 as they're seen as hardcore gaming consoles; not the innocent-looking Wii. MS and Sony can try their hand at it, but without seriously awesome must-have games exclusive to their motion controller, it'll flop. Harcore gamers won't be fooled.
Hope i'm wrong, though, and it's a huge success with a motion-control Gears and God of War....
