cause microsoft literally does not have any incentive to improve it, or even to have all of it work. Currently the mainstream computer market consists of people who do not really understand how to use computers, which is just another way of saying they dont know how to troubleshoot at a basic level, which rules out linux for them. (linux can do just about everything most people need, but usually only after a bit of fiddling, and people get scared of that) which leaves windows and osx. osx doesnt support a lot of games and in general sucks for people who like computers (as opposed to people who like what computers can do, for which osx is pretty good I guess) which leaves windows.
>>tldr: windows is only used because everyone uses windows already. It literally has a 100% or near 100% monopoly on its specific use case* This will not change until software developers, particularly game developers, particularly mainstream game developers for hit games and AAA games, start making their products available on linux, either by ensuring that they are functional with wine or something, or by natively compiling their code for linux as well as windows.
* In other words, if you want to play games, you cant use osx. If you want to be able to play games, do web browsing, office/office like documents, all without having to figure out how to make it work, you have no other option besides windows in almost all circumstances. The exception would be if the specific game you want to play is playable on osx or linux, either with wine or natively.