Desktop Linux is still hovering around 1% market share like it always did. It's not a big potential userbase, and most of it probably doesn't have the drivers or even hardware to run modern video games.
True, but pair that with the first part of my response and it makes sense IMO. If you're making games for Linux, why not make them available to desktop users too? We don't know either way if the Steambox will be a hit. A venture into the console market is always a risk, but if the likes of Sony and Microsoft can pull it off, the cards aren't off the table for Valve either.
As it stands, the Steambox is on target to have ~80 launch titles, assuming all developers commit in full before launch day. This could be as high as a hundred, but definitely more than the 49 games currently available to desktop users. Not a bad start if you ask me.