According to

Shigeru Miyamoto has wanted to incorporate the co-op play feature into the Mario series for years now, but it's only now that that wish will come to fruition in the New Super Mario Bros. Wii, all thanks to the Wii's "graphical capacity."

In a Q&A session with investors, Miyamoto explained how this becomes possible:

It was all thanks to the graphical capacity of Wii console. If two players or more are simultaneously controlling Mario and Luigi in the same screen on a platformer game, the slower player will be left behind by the faster player, beyond the game screen. This time the screen zooms in and out accordingly, thanks to the capacity of the Wii console. The camera can zoom out as far as three times wider where you can see a very small Mario running around, and zoom in to show huge Mario and Luigi. The camera will work automatically and simultaneously, according to the position of four players. That's how we realized a Mario game which can be played by up to four players at the same time.

Miyamoto also cited the multiplayer mode, saying that it actually helps balance the game for different skill levels. According to him, it only takes one veteran player among the group to lead the other players in the game. "People can enjoy a kind of play like peeking inside Bowser's castle to see how scary it is."