I can explain how it is in my immediate family circle and maybe that can be extrapolated further.
Starting with me, I use linux laptop for work and windows desktop for home. I find linux to be a much better OS for doing development than windows, however, being an avid gamer, I had no choice but to stick with windows at home.
I tried to switch my farther to linux on his laptop, however, he always finds some obscure application for his audio setup at home (borderline audiophile), which only barely functions under windows xp. It was just simpler for myself to leave him be.
My mom does not mind the OS, in fact, she does not even know what OS is. However she constantly switches between the desktop and farther's laptop and has trouble working with two different UIs. So now she is also using windows XP on desktop.
My sister is studying at the university and naturally has to write a lot of documents, Most people use MS office for that and interoperability between libre and ms office suits is just terrible. She also said, she prefers Microsoft one over Libre. As you might have guessed it, she is using windows now.
I convinced my cousins to try linux once. The very next day, they realised that most of the games that they play don't work under linux, they switched back.
My grandmother's computer is looked after by my cousins and they are less interested in computers than myself, and since they use familiar windows, grandma uses windows as well.
One of my colleagues at work actually likes windows 8. Everyone can have their opinion right...
Company actually has extremely terrible linux support for tools. I actually have to figure out ways to make things like VPN work myself, as IT support can't do it for me. So other colleagues, choose to spend time working on code, rather than fiddling with linux config files. As far as I'm aware, most don't have desktop computers at home, other than work laptop. And like my cousins, they are probably less interested in having linux at home, so their children are using windows on their computers.
tl;dr Linux has poor support for pc games, popular software suits and niche consumer applications. It is also unfamiliar to most users and therefore complicated to administer. Finally, most people have other interests than operating systems and windows fulfils their needs just fine.