Wii Sports does not sell. It is bundled software. People didn't say "Hey like looks like a cool game, I'm going to drop $60 on it."
They said "Hey I'm going to go buy a Wii" and they got stuck with Wii Sports in the transaction. If it hadn't been bundled it wouldn't have sold very well. It's essentially a training guide on how to use and control your Wii, that's pretty much it.
How many people with 5+ games for their Wii say "You know what, I'm going to play some Wii Sports" on a regular to semi-regular basis, such as COD4 or Halo?