The biggest factor was that the OS was written to run on minicomputers as opposed to big iron, and was written in a portable language. Thus it could seamlessly jump over to micros (as soon as these became powerful enough, of course), and even later on to embedded and "wearable" compute. You just can't do that unless you're writing in a highly flexible and highly portable language - more like the FORTRAN of systems languages than anything like JavaScript!
Then came an OS, with a symbolic price instead of the typical market prices of competing OSes, alongside source code tapes, and a systems programming language that was the "JavaScript" of system languages.