The ancient history is that mainframes (mostly IBM) were so expensive (upfront + license/support) that many places jumped onto minicomputers starting in the 1960s and that process repeated itself a generation later as hardware continued to get cheaper and (later) Unix went from a curiosity to something a lot of people wanted and was generally both considerably cheaper and — especially appealing – meant you at least in theory weren't just locking yourself into yet another proprietary single-vendor platform.
For awhile, that meant companies like Sun, HP, DEC, Data General, SGI, etc. would sell you their hardware running something Unix-like and this was popular enough that even IBM started selling AIX to avoid losing customers entirely. At some point this was termed “Open Systems Unix” since the theory was that you could mix vendors fairly well, although anyone who worked in that period could tell you that the definition of “Unix” or “C” had significant variations across vendors.
As x86 improved, that cycle repeated again. Unlike its predecessors the 80386 was 32-bit and had hardware memory management and permissions so it was much easier to maintain the Unix model there, and the sky-rocketing volume meant other hardware platforms found it increasingly hard to remain competitive on performance or cost. One by one they either gave up and ported to x86, left the market, or tried to specialize in some niche like SGI's various high-performance computing products with exotic interconnects, graphics hardware, etc.
(Disclaimer: I work for IBM Power Systems, opinions my own)