Opinions are great, and all, but the answer to "why" is because Steam is creating its own console which will be built on Linux, and this is a) a great side benefit for those of us on PCs, and b) a huge QA opportunity.
It's not like Linux gaming hasn't had a chance in the past. Back when the choice was the relatively crap Windows ME, and Red Hat was making a real push and was still free, ID and a few other companies released Linux versions of games, that you could actually find at the store and buy in a box. It's never been a Windows vs. Linux thing.
Truth. Valve do not care about linux users at all, the whole Windows 8 is bad thing is just a complete bull crap scape goat reason for trying to cover up the fact they want to have their own gaming machine, which will have to run linux.
Due to Windows 8, and the quality Linux Distros coming out, I think more and more people are taking the plunge. Currently the major hold back is gaming. If my games ran on Linux without issue, I would likely be using it as a daily driver. I don't like/agree with the direction Microsoft is going, and it's unfortunate that the only way to have my computer as I want it, is to stick with an older version of Windows.
That, in my opinion, is why.
Linux is great for a lot of things, namely hacking, however it is not good at gaming nor will it ever be. Drivers, support, market share and above all -fragmentation are going to continue to hold it back. Microsoft have not done anything to stomp out steam, so I don't know what you mean by the direction they are headed because last time I checked, steam was running on this Windows 8 box just fine, exactly the same (if not faster
) then it was on Windows 7.