In the 1980s, XENIX 286 was able to use the 286 protected mode to deliver real protected memory and pre-emptive multitasking on a PC-AT. That more or less guaranteed a certain market share, because a PC-AT was a lot cheaper than anything with a 68k or a 386.

Xenix was successful because it delivered a lot of bang for the buck. It ran on relatively inexpensive hardware and delivered a lot of UNIX flavor despite the limitations.

Really early versions of Xenix would run on an actual 8086, with no protected memory whatsoever. I don't think any other UNIX ever managed that.

As for 8086 and MMU-less unix systems, xenix is certainly not unique in this. In 90's there even was heavily modified Linux kernel that ran on such systems.

And as for 286, there were (and probably even still are in production systems) multi-user multi-tasking CP/M derivates. Two large niches for such systems were industrial automation (IIRC some versions of Siemens Simatic ran that) and point of sale (IBM).

https://github.com/jbruchon/elks

But it's still pretty unstable. If you have a spare 8086 and want a Unixoid, go look up Minix 2 (not 3). It's not very comfortable on a 8086 with 640kB, but it works. Proper filesystem, processes, Bourne shell, the lot. It even has a self-hosted C compiler. You can get real work done on it (slowly).

https://www.minix-vmd.org/pub/minix/2.0.2/

Later versions really want a 286, but that gives you proper memory protection too.

Of course, if you want any processes bigger than 64kB code / 64kB data, you need to upgrade to Minix 386.