Because what do you do next? Five years from now, when the PS4 and NextBox show up, they're going to jump in hardware power again. And then Nintendo's left with a machine that looks two generations old instead of one. The motion controls, now considered somewhere in between "the best damn thing that's ever happened in the world" to "gimmicky stupid childish nonsense," will be super played out and exploited. Unless there's some other gameplay innovation on the horizon, Wii could be viewed as a fad, susceptible to the same fickle emotions that killed snap bracelets, pet rocks and Sega. And if Nintendo bites the bullet and gives the machine a visual kick in the pants, well there goes its whole mantra that graphics don't matter. There's just enough steam with this idea to last one generation, and none after that.
