Well... it's *really* obvious why Games for Windows failed, it's because it does nothing for the consumer and just plain sucks.
Second, Steam is a very useful tool to use for digital distribution and management. It has a nice interface, good download speeds, awesome deals. And while at launch it's prices are generally just MSRP, with no ~$10 sales you get at Best Buy or some other places, their weekend and midweek deals are often amazing. My backlog of games has tripled since I started using Steam (near exclusively, now).
I don't think they have an unfair monopoly, and even if they do, it's completely the result of no one else stepping up to the plate. Granted, I think Impulse will take off in a few years (which'll be great), it's just not on the same level as Steam (yet).
Oh, and Direct2Drive lost my interest completely since I can't use my Oblivion mods with it.