[0815test]

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!

[pjmlp]

Ironically, systems written in hardware 10 years older than PDP-11, thus with less resources, were written in safer system languages, go figure.