There are three basic reasons why I don't use Linux on my Desktop:
1. Software
I use particular software because a) I like it and b) I've paid for it. a) is the biggie here. I like FeedDemon to read RSS feeds. I paid for it. I like it. It works well. And it isn't on Linux. Indeed there is no equivalent for Linux. This is only one. There is other software that, quite frankly, has no acceptable analogue in Linux. Also, why should I have to learn a new, often substandard, application in Linux when what I know and like is already working in Windows?
2. Hardware
Specifically, drivers... Ever tried to use dial-up with Linux? Know how damned frustrating it is to be told "Nobody uses dial-up anymore." and then dismissed? I do. And I was stuck on dial-up until *this year*. Yes, I went through all the various websites, tutorials and FAQs and still had a helluva time figuring it out. Contrast with Windows where it just worked without my having to hunt down settings to use, changing MTU/MRU values, or figuring out chat scripting. Same thing is happening with my Sierra 3G/4G modem (250U by model number). Doesn't work in Linux. Searching the web, fora, FAQs leads to frustrating and contradictory "possibilities of getting it working." On Windows, I installed the software, rebooted, plugged in modem and was up and working.
3. Linux is just not ready for the Desktop
Until you can just plug & play with Linux as you can with Windows, Linux just simply isn't ready for the Desktop. Linux is a wonderful system and, frankly, I'd prefer it for nearly any other use OTHER than my Desktop where I just want things to work, not get in my way and not make me hunt down ways to make it work. Just work.
In the end, I don't find any OS better or worse than any other. Linux, however, is damned frustrating to use when you want to just work and not have to re-learn things you already know/like/prefer. /D