You've heard this before: third parties claim they can't invest in Wii because Nintendo's first-party titles sell too well, Nintendo claims third parties just aren't trying hard enough. Sounds a bit like the third parties are whining, doesn't it? Start crunching numbers for Wii software sales, though, and it looks like the third parties have a serious point.
Usually game sales work consistently with the business law called Pareto's Principle, which states that 80% of all effects come from 20% of all causes. To put this in game industry terms, generally 80% of all software sales for a given system are generated by 20% of all games released.
This principle holds for the 360 and PS3 market, but not for the Wii.