My wife and I are a perfect example as to why the Wii is trouncing the rest. I'm 36, and I used to be an avid PC gamer. My wife has never been much of a gamer, though she does like some of the zelda games. Now with kids etc, we just don't have the time to waste on games like we might have before. I don't want some expensive "do it all" machine with a zillion options and complicated RPS games. I want a simple, easy to use fun game we can just play when we get a chance. The Wii does just that and more -- it has an innovative controller as well.
So, what did we end up buying? The Wii of course. Ditto for everyone else we know in our age range. Nothing wrong with xbox 360 and the like, but the market for a much simpler game console like the Wii is waaaaaaay bigger.