Compatibility, getting things done, makers of peripherals not being linux friendly. All those reasons. I use Linux on my desktop and have for a generation. It is better than ever. Even now, to some extent, printers or other gear can be a little problematic. It isn't as hard to adapt as most windows users imagine, but there is an adaptation cost. People don't want to become computer scientists, they just want to get things done. I don't know, but from what I see the % of people owning a PC with Windows, LInux, BSD, or MacOS has probably peaked. People are now more on phones which were made from the beginning to be more of a mass market computing appliance.
Even I could manage with no computer more than my phone if it were easier to interface it to a full keyboard and monitor. People with phones just get an app and don't need to know anything more. Windows is trying (and failing I think) to turn their machines into the same thing. Get it from the Windows store needing to know nearly nothing and use an app. MacOS sort of has backed into the same idea in a somewhat more accepted way. With both I don't feel like my computing machine is mine the way it has always been with Linux, was for a long time with Windows and never was for MacOS. Macs are expensive, and you have to pay for more software with it.
I understand your frustration that Linux isn't utilized more, but there are lots of dollars behind Windows and MacOS. Worst of all is Android sort of grew from Linux and cheated on the licensing of open source material. I feel like both Jobs and Gates were snakes in the grass.
I need to replace my decade old desktop. It is functional enough, but I could get something faster for the next decade. I'm trying to decide between building a Windows machine, or one for Linux or get a Mac Mini. The latter is easier and plenty powerful enough. But I like that OS the least, and the prices beyond the basic memory and HD are obscene being nothing more than a rip off.