The Wii is taking on a part of the market the PS3 and XBox originally didn't, and it's because of that it's been successful. Still, because it's less powerful than the PS3 or 360, it's not seen as a platform for the hardcore gamer.
The Problem for the Wii now, is that it's the "1hr fun time" system, while when people want to play a more hardcore game they will turn to the 360, PS3, or PC. Even if the Wii started targeting hardcore gamers, it's a hard sell if it's sitting next to a PS3 or 360 when those systems are constantly putting out graphically superior games.