Or POSIX to be correct.
At the beginning there were mainframes and minicomputers. While they did run various operating systems, they were all kinda UNIXy as far as I know. Only noteworthy exception that comes to mind were LISP machines. I have yet to try some Open Genera, but they didn't caught up and needed spscialised procesors that I'm not sure how usable were for non-lisp stuff. But at least they gave us emacs. (which I also have yet to look into)
When home computers moved from BASIC or FORTH to some actual operating systems, UNIX was already a thing. BSDs were becoming popular and even Microsoft tried to make UNIX-like OS called Xenix. Home computers were not that strong however and they didn't really need multi-user support either. This allowed operating systems like CP/M to exist.
When IBM decided to make the IBP PC, they originally wanted it to run BSD, but there were legal problems happening around BSD at that time (which alse caused the GNU project to start) so they asked Microsoft to make something else. Microsoft didn't want to have anything to do with Xenix or something, so they bought CP/M inspired OS callse QDOS from Tim Paterson and remake it to MS-DOS. Since IBM PC was made from publically accessible parts, IBM PC clones were made and they ran either MS-DOS or some other compatible DOS system. DOS then evolved into Windows, and while always technologically inferior to it's *nix counterparts (for example no virtual workspaces until 2015, while Xorg WMs had them from the beginning (and different TTYs were possible even before, which could also count as simmilar concept)) it became the most popular desktop OS.
This is where the list of non-POSIX systems end. Apple eventually switched to Darvin (BSD) as MacOS base. Android is Linux, gaming consoles use BSD nowdays (or they're Steam Deck and run full Linux desktop), even more niche OSes like Haiku or SerenityOS are POSIX-compliant. If Windows ever falls out of use, there will be nothing but *nix and I don't think that anyone will be able to gain success with any new idea. It's hard to port software between say Linux and Haiku, now imagine how hard would it be to port to some OS with completly different design. No corporation would be onteresting in experimantal OS with no software and hackers will porbably stick with POSIX, just look what happened to Plan9 that just tried to make UNIX a bit better. Only reason why was DOS able to succede is because there was not enough software on one desktop platform to hold people from trying new ones. We have advanced too far to be able change.
I don't want to say that it is bad by any means. It is state that had to be reached at some point and POSIX is quite good ending point. I just feel it's at least a bit sad, that many possible OS designes didn't get their chance to ever see the light of day. I wonder what could happen if LISP machines became popular or the designers of first operating systems had different ideas on what should OS look like.
At least it allows us to tinker with other parts of our systems, unlike some other alternatives...