The reason is: the library sucks. The Wii has a reputation among the critics of being home to an unusual amount of shovelware (low-budget, low-effort shit shoveled out to take advantage of dumb kids and parents) and I believe this reputation is well-deserved. Of course, a good system is going to have lots of shovelware but it will also have tons of classics to make up for it. HERE ARE THE WII-EXCLUSIVE CLASSICS . . .
plus a short downloadable game: Lostwinds
And that’s it.
There are definitely many other good games for the system but they’re all available in either near-identical versions on PS2 or superior versions on PS3/X360. For instance, The Legend of Zelda: Twilight Princess and Resident Evil 4 are both available for GameCube. Bully and Okami are available for PS2. I really can’t recommend purchasing a Wii over an Xbox 360 unless you have a major hard-on for Nintendo (like I do).