Gaming is one of the last legitimate refuges of Windows dominance. There are a lot of us who wouldn't care about (read: would be willing to switch) their OS if not for PC gaming.
Now TF2 runs on Linux, and the perspectives for Linux gaming are very good in general, but I no longer want to switch because I am happy with Windows 7, I hate Unity and Gnome 3 and their dumbed-down UIs that make me want to bang my head against the wall, XFCE is OK but feels like going ten years back in time, and KDE 4... well, that one is not too bad, but I still prefer Windows 7.
It's a pity that all this Linux gaming surge didn't happen back in 2008 or so, when Windows was in its lowest moments with Vista and most Linux distros hadn't yet been ruined by overbearing UI visionaries. I'm sure that desktop Linux market share would have soared in that case.
What about pretty much all larger businesses in the world? Windows sure dominate the corporate space.
When I buy a computer with an OS, I tend to pick Windows mainly because I intend to game on it. If I don't intend to game on it, then OS would stop being a major factor in my decision-making. I might spring for a Mac instead, or I might get a Linux box. It's no longer the point.
If I were buying a computer for surfing the web, I can get what I want. If I were buying it for word processing, I can get whatever. If I were buying it for photo manipulation, I can get whatever. For Door #1, I'd be looking at network connection options. For Door #2, I'd probably be looking for compactness. For Door #3, I'd want graphics and RAM, I think. For none of these is my primary consideration, "Well, the program I'm going to want to run isn't going to be available unless I get Windows."
That primary consideration exists for gaming. If I bought a replacement gaming rig today, I'd get a Windows box. Period.
Two years ago I would have said another big reason was Outlook, because no other desktop calendaring app comes close (even though it sucks hairy balls as an email client), but nowadays web-based solutions, particularly Google Calendar, are as good if not better.