I think it is somewhere around a 50 - 50 split with the games.  Sure, motion controls were a exciting new way to play, but without compelling software, they're useless.

Motion controls and how it was used in Wii Sports was a big draw, it broke down barriers to gaming and combated nonconsumption, it made gaming something the whole family wanted to play, then other Wii games added to the draw

This quote from Wikipedia answers alot. Just replace GC with Wii,

"Yamauchi also wanted the machine to be the least expensive available of its kind, in his belief that people "do not play with the game machine itself. They play with the software, and they are forced to purchase a game machine in order to use the software. Therefore the price of the machine should be as cheap as possible." Nintendo hence priced the GameCube significantly less expensive than its rivals in the market, although the console's games were priced identically to those designed for the competing systems."

http://en.wikipedia.org/wiki/Hiroshi_Yamauchi

Hmm interesting responses. I wonder if this could be the source of the 'birdman' talk from Malstrom. Essentially other consoles copying the concept of the motion controls without truly replicating the reason why they were truly popular.

Not to be a Sony or MS hater, but it is a trend for hardware releases mid-lifecycle to fail.  Definitely for a concept that people have already bought Wiis for, even if the other product is better.  

not just motion controls, its to do with the percieved price and obviously its marketed in a different way, specifically towards youngsters.

I think it was the games, whether they were Wii Sports, Monster Hunter 3, Tatsunoko vs Capcom, Zelda, SMBWii or whatever

I think it's the cool casual sports games, the Nintendo brand games, the ads, the hype, the pop culture, the motion controls and the popular add ons (balance board, guitars, guns, drums and so on) mostly.  Price likely plays a part in it too.

for me...

motion controls was the main draw of why i bought a wii.

the part where a majority of the wii's main library doesn't actually use or rely on motion controlls is what turned me from a nintendo fanboy into a nintendo hater.  $450 worth of hardware later the only thing the nintendo sold me was a broken promise.

Ouch. I feel for you. So thats one Wii 3 remotes?