NT was designed to be CPU and OS level API independent, of course mainly being used with closed source software meant hardly anything supported non x86 CPUs, and if you wanted Unix you could just run Linux by then
The OS-level was much more CPU agnostic at the time and programs would aim for an OS rather than an OS/CPU combo.
Linux preserved that for much, much longer than many others (the only other big name was NeXT into MacOS).
Now with ARM finally "catching up" we see it growing again; nothing is new.