Is it the install?

It's as easy as 1+1=2, there are countless tutorials online, I did it in one afternoon... I had a little problem though with the BIOS, it didn't want to accept the OS as a OS, had to allow it through the BIOS with a superadmin password,etc..etc... But it all worked fine and I'm really happy with it...

Is it because you absolutely need a certain software that only runs on Windows?

I played countless windows games with wine some years back, it's really easy to use, and even some paid programmes for windows have a replacement in linux that is free to use... example: Gimp is free to use open source and it is just as good as Photoshop if not better...

Is it because Linux is too difficult to use?

Linux mint looks easy to use and is really good for old computers, much better then any windows!

I think most people use windows because everyone uses it, and it's not like they are given many options when they buy a PC, most PC's already come with Windows installed, why would someone bother to install linux. One thing that life taught me is that people would rather be comfortable instead of struggling to be a little better than comfortable, windows is comfortable and Linux is a little better than comfortable.