I suppose I can technically answer this. I do use Linux full-time now and have for several years, but prior to that I had a few false starts where I'd switch back to Windows. Usually it was because I'd encounter some technical issue I just didn't know how to fix besides reinstalling the whole OS, or a graphics driver issue. For example, at one point when I had an NVIDIA graphics card only the newest drivers from NVIDIA's website supported it but the 'stable' drivers in Ubuntu's repo didn't, so I had to manually install the drivers. Except then whenever the kernel was updated by Ubuntu (basically every week) my display stopped working and I'd have to switch into a TTY and manually reinstall the drivers.

Now I know how I'd fix that (setup some rule to reinstall the drivers whenever the kernel updates, which I believe is now the default anyway), or use a PPA containing the latest NVIDIA drivers, or use AMD instead - but really any kind of problem that requires the user to both diagnose and fix the issue prevents non-technical people from adopting it.