Modern C is very different to K&R's C. Which itself was based other programming languages before it (like B - got to love their naming convention for programming languages!). But the history of C aside, it wasn't the only language that operating systems were built on. LISP, Pascal and obviously assembly / machine code too. In fact Pascal was a very popular systems language on home computing in the 80s and early 90s. If I recall correctly it used heavily by Microsoft and Apple too.

Don't get me wrong, I do like C. But it wasn't the run away success nor holds quite the monopoly you suggest it does.