In the early 80's it was Bill Gates who made strategic decisions for MS. That was even before they went public. My wonder is if Gates had ever used Unix. He (personally) developed BASIC for a CPM (I think) machine. I am unaware of any system level skills in his experience. If he had knowledge of or used Unix or XENIX (for which he had a master license from AT&T), why on earth would anyone go down the bazaar path of DOS with lettered drives, tortuous IO interfaces, and assembly language source code? Why didn't he choose a far simpler to support and easier to learn operating system that had 10 years of maturity. I would love to hear Bill Gates' description of the development of a DOS over Unix strategy.
My guess is there wasn't enough memory on the first IBM PC's. I worked with LSX while at BTL and forget the memory footprint of LSX. Memory protection was another thing, but LSX looked and felt like UNIX without memory protection. Does anyone recall how much RAM memory could be put on the first IBM PC's? That was probably a major problem.
My memory of the LSI-11 architecture has faded. Same for 20286. In the early 1980's I had never heard of Xenix.
Ed