There were tons of almost-PC-compatible manufacturers back then, selling their proprietary very slightly better, but incompatible systems. They'd have a slightly faster but incompatible bus system, slightly higher capacity floppy disks with a custom format, a proprietary interface that was quicker but only worked with their own printers, a higher resolution display or with more colours but which required special software to take advantage of it.

The Unix workstation market that NeXT, Sun and SGI competed in was a bit different and didn't really overlap much with the PC market.