No discussion of the history of Unix, no matter how brief, should fail to mention the influence of Multics, from which it took numerous ideas. Check out the Multicians web site (I’m not allowed to post the link) for more information.
It was only when Bell Labs pulled out of the Multics project in 1969 that Ken Thompson built what he at first called Unics.
That first version was implemented on a PDP-7, in assembler, and it was when porting it to the PDP-11 that he realised that they needed an implementation language with at least weak typechecking.
Incidentally, the DEC PDP-11 was a minicomputer, not a mainframe.
The reason so many universities started to run linux was twofold: firstly, it was superior to DEC’s proprietary OS, secondly, as Bell at this pre-anti-trust-case time were not allowed to sell anything computer-related, educational establishments could get the system complete with source code for some trivial amount, $100 rings a bell.