There were plenty of other operating systems. What happened is that computers changed. Unix (which itself descended from the earlier operating system Multics) was in the right place at the right time to both drive this change and be driven by it in a hardware-software dialectic. As mainframes yielded to minicomputers and then to microcomputers, operating systems built for those systems, like DEC's VMS or IBM's TPF, largely went the way of the dodo. They're still around for certain uses, but the vast majority of developers will never work with one.
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