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....

If the Wii had a controller like the 360 or the PS3 it would have been burried.

Motion made the Wii what it is today

I think the deeper issue here is that people are having problems separating the hardware from the key software.  And really, that's partly true, as Nintendo doesn't tend to make distinctions there either, with their hardware and software teams fully integrated together (which is vastly different from Microsoft and Sony's setups).   Wii Sports and the Wii remote work in concert, but the harsh truth is Wii wouldn't have been the phenomenon it is without either of them.

PS3 expensive

Ninendo loyals

cheap price

hype and nintendo marketing

Agreed with Solid_Snake4RD. If the PS3 was cheaper and had better titles available at launch, the PS2 user base would've bought PS3s instead of Wiis, meaning that the PS3 would have won this gen. Needless to say, Nintendo fan boys will always buy Nintendo systems, so that helps them in sales. PS3 marketing was awful until Kevin Butler came in, while the Wii's marketing was top-notch. Cheap price also helped. 

That's it. Plus it was the right price.

It's about the mindset that came with motion controls. It's about far more than accessibility, as Kinect will soon learn the hard way. It's about an environment that molds itself around great software ideas, not about software molded to fit the environment.

The idea of Wii Sports predated the Wii, in all likelihood.

what made Wii and it's 10m sellers, is the ability of Nintendo to combine CONTROLS and style like no other

if yout think of it, they only thing that really gave Wii the success GC never had, was the controller! (the basic difference between the two)

Nintendo keeps Mario the top video-games character for how many years now, they have their own way to make their games unique without the HD graphics, they are much more creative than Sony and Microsoft

Sony and Microsoft have no new ideas (in case somebody didn't notice), they jusy copy Nintendo and try to expand their ideas (as by the time they copy, technology is better and cheaper),  but they don't make it as good as pleasant as Nintendo

Mario Kart Wii was much more successful than Double Dash because it has the motion controls and the style of Nintendo (it does not use motion controls? anyone noticed the wheel)

Wii Fit uses nothing else, except motion controls, my opinion! Twilight Princess, Wii Sports.....

Wii is noting without motion controls, except some games that only Nintendo can make, SSBB!

no, Ps3 is Ps2 with better graphics and worst price

the experience is very similar (for average human beings), but the experience between Wii and Ps2/GC is very different and obvious , and that's what made Wii sell

Launch titles can have great impact on a platforms success, or not. PS1, PS2, and DS all started out pretty slow. PS3 could have used an at-launch killer app, but that wouldn't necessarily have given it momentum.