It will eventually, but not for a year or too. M$ must wait for (a. many users to upgrade to Vista and (b. they like to encourage people to invest in a 360.*

*some will argue me on this, saying that it makes little sense, but it does. if I own a system, i'm not gonna own it for ONE game. I'm gonna buy a few, if not many games for it over time. For example, (i know little about 360) buying my PS3 didn't actually net Sony more than a few bucks. The cash cows on a system are games. M$ needs people to buy games for the 360 to make the 360's division of the company profitable. M$ doesn't need to sell PC games to make a profit on windows.