As long as you're content with very slender selections of 3rd-party software and hardware support, these other OSes will do a fine job. And it's true that the people who find Windows confusing to install and administer would absolutely freak if they had to set up a Linux derivative all by themselves....
Windows is by far the runaway best-seller that it is (even though many Linux distros are free to the end user, the demand for
purchasing Windows is astronomically higher nonetheless) for what I think are essentially three basic reasons:
1) Windows supports far more 3rd-party software.
2) Windows supports far more 3rd-party hardware.
3) My current @home version of Win8.1x64 (non-Metro) installation is far more backwards compatible, which sort of loops back into #1 above, but is actually a different factor in and of itself.
You could of course add that setting up Windows is far easier than setting up your average Linux distro, but a lot of that is because of #2, above.
Bottom line for me is that if you only use a computer OS for web browsing and email and some amateurish word processing & spread-sheeting, then probably any old functional OS will do provided it can support the hardware you own. But if you actually want to *use* an OS to accomplish a wide variety of tasks over and above web browsing, etc., there's no equivalent to Windows anywhere. There is little mystery behind Windows' popularity, imo.
I recall years ago coming off of eight years of the Amiga's Workbench OS versions (because C= went belly up), buying in a sequence no less than three separate versions of OS/2. I was very disappointed that even with each successive version I managed to have at least one piece of critical hardware which no version of OS/2 supported (IBM had not even written placeholder drivers for it as Microsoft does)--on one occasion that I recall it was my CD-ROM drive, for goodness' sake. End result was that after much finagling I couldn't do what I needed to do with any version of OS/2 and wound up successively installing, then uninstalling each version! IBM simply quit with the OS/2 job only half finished--OS/2 never had a chance. Belatedly, I moved my current OS at the time to Win3.x which I pretty much loathed because it was so crude, at least compared to C='s Workbench. But then Microsoft surprised me with Win95 which was a giant leap ahead from Win3.1, and then to XP which was finally getting into close to the "decent" range. And with Vista, Win7, and now Win8.x, I'm very happy with Microsoft in the OS department these days. Except for the Metro GUI--I don't have any interest in touchscreens (but fortunately Metro is optional in 8.x). I think if Microsoft will get back to desktop OS development with gusto the company has a lot of great stuff in the pipeline almost ready to see the light of day. The pent up demand for new and invigorating desktop functionality must be enormous.