Even as a Wii fan, I don't understand how it outsells the competition by so much. It didn't really have any big game as mentioned by youjiro and the Xbox 360 had a major game release along with the fact you can get the 360 cheaper or for little more than a Wii depending on what version you get. I had been thinking the PS3 would eventually pass the 360 but as the weeks continue to look like this, I don't know.
I'll come up with something better eventually...