The Wii had the Rock Band games too, though.
It's not about "which system doesn't have which game" or "which system has the most local multiplayer games."
You guys are kind of losing the point about the relationship between local multiplayer and Nintendo. I, personally, still consider my Wii to be the go-to local multi machine, but I've already said that all the systems have vast arrays of games that support the function. Nintendo in particular re-invigorated local multiplayer, putting the emphasis on four-player simultaneous play, as well as one- or two-player games that are allow for easily switching off and sharing between players, so that no one feels excluded. An early game like WarioWare supported up to 12 players at once in its multiplayer mode because you could just pass the Wiimote around. This was happening while other companies and other games were omitting local functions in favor of online ones. Obviously the situation has gotten better, but Nintendo was there to keep same-room play alive when it was looking bleak, and their efforts have been greatly influential. Sadly, they sacrificed having strong online functions, but at least they held up one side of the equation. Let's hope they address the other side better with the Wii U.
Everyone has something to offer in the local multiplayer realm, but Nintendo is associated best with it because they put so much into it from the get-go. At this point, their experiences are entirely distinctive from anything else, even with all the copycat software out there. With Wii U, Nintendo's once again changing things up by adding the Gamepad to the mix, and this will re-invigorate local play for people (especially considering 5-player support). Nintendo with the Wii U is just adding their own twist to everything, so that the wheels of both untraditional multiplayer (Wii, Kinect, Move) and traditional multiplayer keep on turning.