I don't know--I don't use Windows 8.
I use Windows 7. I don't need 8 because neither my desktop nor my laptop have touch screens, which makes using 8 entirely pointless.
Yes, Steam is heading toward Linux support, but they haven't reached it yet. Further, GOG.com has no Linux support as far as I'm aware. Until either of them do, you can expect people to not bother with Linux if they are at all interested in games.
Also, I would think the biggest part of the PC market is in office computers. It's probably the reason why Microsoft Office and Adobe Photoshop are so popular and expensive--because businesses are willing to pay for it (likely everyone uses Office because everyone else uses Office, alleviating problems caused by converting file formats.)
I'm not sure how keen office managers are on switching all their workstations to Linux. Servers, yes, but workstations?