Microsoft "answers" the PC question (Here ABF)

[Weltall]

Because Microsoft doesn't control the PC platform and cannot collect royalties for games designed for its use. On the 360, it doesn't matter whether Microsoft makes the title or not; they're taking a mafia-like cut of every sale regardless.

The reason why Microsoft favors the 360 is because it offers a stable opportunity for return on investment. The PC does not, not as a gaming platform, anyway.