I don't know how the wii could be on top either than it is cheap, people are buying it to merch them, cause in some areas, wii's are sold out, and it may take months before new shipments come in.
And, people buy them, try them out, and they dont like it after they buy a 360 (happend to 8 of my friends so far). Plus, the wii has no games, so theres nothing to play it with.
And, the wii is kid-friendly, meaning parents will gladly spend $250 on buying the wii, just to play wii sports (free) and wii fit.