Because of the application barrier. It's a chicken and egg problem, nobody wants to use an OS without applications and not many want to develop for an OS that n

Because of the application barrier. It's a chicken and egg problem, nobody wants to use an OS without applications and not many want to develop for an OS that nobody uses. To get it started, a lot of resources and power (e.g. quasi-monopoly or tons of money) is needed, or you need to have some binary compatibility with another OS. The latter is difficult due patents and copyright, and because it will likely make applications slower than on other platforms. Apple was able to do it, because they sell hardware, not software, but the transition from Classic OS to OSX was hard and rocky even for them and could have failed. It's a pity, because neither Unix/Linux clones nor Windows are really good and technically sound desktop operating systems from a modern perspective, and it would be possible to develop something way better. As for mobile, I don't know, it's a different thing.