I think it was the price more than anything. Both of the software license (not cheap!) and also the hardware requirements. It's something that doesn't come up much anymore but RAM used to be one of the main limits of a personal computer. A/UX required a high-end Mac at the time. And Macs were already high-end PCs. And UNIX wanted a lot of RAM. 16 MB was barely comfortable in the early 90s. This was a time when when typical entry-level Macs were still selling with 2 - 4 MB sometimes and a full 32 MB upgrade would cost twice as much as the base model.

In short, basically the same reasons we didn't all run SCO UNIX or whatever on our IBM PCs. Much the same dynamic for why the Windows NT kernel took so long to come down to home computers (in Windows XP finally). Even OS X's RAM requirements would inhibit its uptake for a few years. "Real" operating systems were too big for the small computers of the 1980s and even early 1990s.