The manner in which it escaped Unix was through Microsoft and Borland, and I spent oodles of time evaluating other choices at the time. I could not get the boss to buy Ada, and everything was a compromise. Borland 'C' was super cheap and 100% available, as was Microsoft 'C' before it.
Turbo Pascal and Apple Pascal ruled back then, at least in Europe. On Apple's case it was even the main OS language.
However with UNIX being adopted at work and universities, many wanted to be able to transport their work between home and work/uni. So people started to slowly adopt C.
I followed the same pattern with C++. Eventually Turbo Pascal wasn't no longer an option, so started using Turbo C++, C was just too primitive and unsafe already back then, vs TP.
At least C++ had some of the safety features I was used from TP, as long as one stayed away from C style coding.
Turbo Pascal was common before Turbo 'C' in the US as well. I personally was rather relieved to change from Pascal to 'C'. It's possible that that transition helped me to understand how to use 'C' safely.
With Pascal, I recall having to use more assembler.
Before C escaped UNIX there were other systems programming languages available.