Kernighan and Mashey mentioned in "The UNIX Programming Environment" (1981): "Success or failure often depends on nontechnical factors, whose importance often goes unrecognized by those who evaluate systems on purely technical terms." And they go on to mention some of these nontechnical factors, such as having started out on the DEC PDP-11.

In Ken Thompson's Turing Award speech, he claimed: "I can't help but feel that I am receiving this honor for timing and serendipity as much as technical merit. UNIX swept into popularity with an industry-wide change from central main frames to autonomous minis. I suspect that Daniel Bobrow (1) would be here instead of me if he could not afford a PDP-10 and and had to "settle" for a PDP-11." http://cm.bell-labs.com/who/ken/trust.html

Similar fate apparently happened due to the specialization of the MIT AI Lab's ITS: "Of course, ITS wasn't portable by any stretch of the imagination. It was an operating system in the old school of development, and it died rather suddenly when DEC announced that they were discontinuing the PDP-10 and its descendants in favor of the PDP-11 and VAX systems." http://www.crackmonkey.org/unix.html

(Personally, I only use a UNIX because it's a tool of the trade. It's free, or built into Macs. I don't enjoy it, but I use it, and found the _Unix Hater's Handbook_ refreshing.) http://www.cs.washington.edu/homes/weise/uhh-download.html