The other issue was performance. UNIX systems at the time needed a far faster CPU and graphics, and far more RAM and disk space. Faster = much more expensive. Which is also why Windows systems were 1/4 the price.
Xenix collector here, I would say they had (sort-of) MS-DOS binary compatibility, as you could run some DOS programs on Xenix 386. I think it's only limited to real mode programs, but I tried it a couple of years ago and it worked.
Not to mention that on Xenix you could develop for both platforms (well, more than that: with the right setup you could target DOS, Xenix 8086, Xenix 286 or Xenix 386), that's quite an advantage.
On a side note, a good collection of Xenix manuals can be found at http://www.tenox.net/docs/.
Sometimes, it even worked!