Because they know a lot of casual gamers have been drawn to the Wii, which is actually a good thing for the industry even though a lot of "hardcore" gamers appear not to like the idea of grandpa playing a bowling game at a party.
So you end up with some gamers even calling Mario games "casual" because the Wii appeals to casual gamers. I guess the guys who regularly rolled over Asteroids at the arcade were "casual" because the game didn't feature lots o' guns and body parts flying in every direction.
Go figure.