In rough order of importance:
1) Games. I am a gamer, I'd rather play video games than watch TV for entertainment. I also find that the games I like the best are either PC only (like Civ), or better on the PC (like Skyrim). So a PC it is. Well, Windows is far and away the best for games. Any other platform has way, WAY less games. So all other things equal, I'd be on Windows just for that.
2) Pro Audio. I like to play with audio creation and production. This is something I could do on a Mac, though not with my prefered tool (Cakewalk Sonar). I couldn't do it on Linux though, the audio production software there is abysmal, and even if it wasn't all the samples I own are Windows and Mac only, and I do not wish to rebuy them, nor have I found any for Linux remotely close in quality.
3) Price. This relates only to switching to a Mac, but to get what I want, that being a tower unit with some good hardware, it would be monkey-fuck retarded expensive compared to PC hardware. I am not a rich man, so while I'll spend a good bit on computers, I can't afford to just blow money for no reason.
4) Hardware support. Linux in particular has issues with much of the hardware I choose to use. I really don't feel like compromising on that, I don't want to have to say "Man I'd like to use that, but it won't work on my OS." Thus far, no piece of hardware I've want has not had Windows support.
5) Ease of use. Perhaps it is just my lack of familiarity with it, or my somewhat odd requirements for use (like pro audio and good 3D acceleration) but I seem to be able to find an unsolvable problem in Linux rather quickly. When I've tried to use it at work I'll find something I can't get to work that even stumps the Linux guys. I feel like I have to fight with the OS to get it to do things, and often the solution is "Oh just write a script," or "Just modify the code and recompile," which isn't an option. I'm not a programmer and have no wish to become one.
6) It works. I'm not big on change for change sake. Were I to move to another platform, someone would have to convince me of the superiority. They'd have to show me what it is I could do there I can't do now, or how I could do what I do better. Even if it is just equal, I've little interest in changing.
That's my reasons at home. At work, well I'm the Windows lead, so of course I use Windows. I need to be familiar with it and be able to easily administer the Windows servers because that's what I'm expected to do.