MS is focusing on the Xbox 360 because it owns that platform. Although it provides the OS for the vast majority of PCs out there, it certainly can't control the gaming aspect as strongly as it can on the Xbox 360. Closed platforms have their risks, but the past few console generations appear to have been exceedingly good at generating revenue (maybe not profit, but MS doesn't need profit at this point, since it receives huges amounts of money from other sources and investing in the Xbox 360 now may see the Xbox 720 as a huge profit driver later).
The fact is that for PC gaming, MS gets paid through virtue of the OS. Now, if they ever need to work more on GFW, it's there, but they get money from Steamworks' output as sure as if it was their own.
Also, there is the prevailing view that PC gaming is pretty stagnant compared to consoles and that the future of PC gaming is MMOs and social gaming. If you believe that, why invest in PC gaming when better alternatives exist?