Could just be the 360 is the best supported of the consoles
The PS3 is more expensive and even a cursory glance shows that it has precious few 'must have' exclusive titles whereas those available on PS3 and 360 tend to look better on the Microsoft machine. I don't know if it's lazy coding or bad support from Sony or the desperate lack of texture memory on the PS3 - but some PS3 games are running in lower resolutions than the 360, have lower frame rates and some horribly blocky textures.
As for the Wii sales falling. Simple - once you've played Wii Sports, what else is there that's worth having? The Wii is a complete bust as a games platform, Nintendo seems to have forgotten to release games for it, preferring a series of gimmicks like Wii Fit and Wii Music which don't have much long appeal and are ridiculously expensive.
I'm on my 3rd 360, whereas my PS3 and Wii have behaved perfectly; but it's clear Microsoft have won this generation of consoles - they've tapped into popular sentiment and got a compelling set of exclusives.