You have to have some software out there for it to sell.
Count the number of releases in the past few months with a 75 or higher in metacritic for the 3 platforms. It will be clear that the only reason why the wii has sold less software, is because less quality software has been released for it.
The playing field is expected to be more balanced next year. 2010 will likely see it tipping in Wii's favor.