IBM lost the Research/Universities to DEC which started out being very open and easy to work with and extremely cost-effective. As more $s piled in the market, DEC started to be more and more protective (and moved more and more upscale). To many at the time, DEC compared to IBM (Mainframe S/360 vs. PDP-6/9/10) again -- worse technology, but 'good enough' (and a new growing customer base). The Unix Workstations come out - again 68K vs. Vax (story repeats). Sun eventually taking the lead from DEC. As Larry points out, Sun certainly started being extremely friendly to the same group -- again cost-effective and leading tech. Sun went upscale and the Intel/Microsoft alliance was good enough to a lot of people.
To your earlier point, Unix lost the developers to DOS, and later Windows, because they were more “developer friendly”.
I think the dominant factor was simple: cost. You could get a DOS PC with BASIC, and later eg. Turbo Pascal, for a fraction of what a Unix system cost. And while the OS barely warranted the name, it was accessible in a way that Unix wasn’t. Over a quite short time, the third-party documentation, language support, editors, tools, etc, quickly outpaced Unix systems, and Windows provided a smooth (and still vastly cheaper) upgrade path.
Unix (in the form of Linux) only recruited a significant audience again when its developer cost (nothing, hard to beat) and ease of remote operation outpaced Windows in the late Internet/early Cloud era.
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For us UNIX historians, we need to be careful and learn from our own history here -- the Cell Phone/Mobile target is the engine for the next Christenian style disruption. It is by far the #1 target for people writing new programs (which I find a little sad personally - but I understand and accept -- time has marched on). In the end, a small mobile target will be the tech on top, and available will be driven by market behavior and those suppliers will be "who has the gold.”
I feel I should point out that both the dominant mobile operating systems are Unix-hased. The UI is necessarily new, but astonishingly the 50 year old basic abstractions are the same.