I'm using Windows. I know; I know--there are better systems out there. Apple has it's fanboys, of course. And, it's great! Except that Macs are also expensive. It's like saying, "Hey, why are you driving a Corolla, when a Mercedes is so much better?" Why? WHY? Because I can afford the Toyota!
And then there are the supporters of Linux and all it's various versions. I looked into it, and unless you're a computer geek with lots of time to fiddle, it isn't worth the trouble. And then there are the compatibility issues. Publishers aren't interested in submissions done on oddball processing programs, such as Open Office, etc.
Besides, I've got computers all over the house operating on Windows, and I don't want to have to learn yet another system. However, I have toyed with picking up a copy of Linux for one of the older machines.