Wii demographic
Reading all the comments so far, it seems very much like the Wii is so popular because it is essentially a "console for girls" (yes I'm aware how much flaming I'll get for this from people who don't read further). I don't say this because it is particularly effeminate, but more because it opens up console gaming to people who would not typically play console games.
Personally for me, the Wii is fun in small doses but aside from "party style" games to play when I have friends round, it's not enough of a gaming console for me (as I AM a long-time gamer). Now I have the likes of Singstar and Guitar Hero for my PS3, the Wii will very rarely get switched on.
Oh and at the 360 fanboy who didn't want his name showing, this was never supposed to be a 360 vs PS3 thread. I accept that both consoles have their selling points, but on a personal level I couldn't take the "jet engine" sound of my brothers 360, so I bought a PS3 instead. The games are coming too, and lets be fair, Halo 3 is quite a disappointment considering they can't even make it run smoothly without crashing on a CONSOLE (no excuses of incompatibility there). Anyway enough about that, they are two "similar in ability" consoles and miles away from a Wii.
Oh and for Christopher, I have spent a shitload more on "real life" than I ever have on my PS3, so your comment is not really accurate is it? While I'm playing my PS3, I'm not bothering anyone else and not spending money on beer. Oh, and if you want to wave your arms around blindly at something that vaguely resembles something else, try getting drunk instead of playing on a Wii.