I mean back in the days they should have been running on IRIX, and SGI switched over to Linux when they made the switch to x86 CPUs. Plenty of movie studios switched over to Linux workstations because of that, porting from IRIX to Linux is trivial compared to porting to Windows, why didn’t the same happen with CAD?
Wintel-PCs for the longest time just weren’t suitable for 3d work, they were office machines.
3d not being required makes a hell a lot of sense and of course it wasn’t people have been drafting on paper for ages. They might’ve ended up on Mac or maybe Amiga, but an SGI workstation is quite an investment when you don’t even need to spin polygons. IRIS GL dates back to the early 80s, doesn’t seem so much to be a timeline but price and need thing. And it’s not like you can’t have a 3d view without acceleration, just would take a while to render and a frame every five seconds might still be usable.
There apparently was an IRIX version at one time but with no user base preference, more likely they were thinking “where’s my C: drive” so once 3d acceleration hit the mainstream everyone happily switched back to Microsoft. Meanwhile you have 3d artists complaining that they can’t move windows with meta+lmb on windows.