applications drive the operating system
I've been running Windows XP on my laptop because there are apps (most notably Quicken) that I want to use and for which I haven't found a suitable replacement. Or that what I've been telling myself; really it's because I'm too lazy to do a Linux install. I know this because now there is an app I want to run which is Linux only and no suitable replacement is available for Windows.
Ironically, the app in question is Evolution, and the reason I want it is that I just got myself a Windows smartphone.