Because it was the language of UNIX distributions (not called that, but it's the same concept).

They went hand in hand. I know lots of people that installed Linux in the 90s so they could use GCC.

If any of its competitors had had package managers as good as some of the modern ones, C would have lost.

well, unix was written in it, making it a natural choice if you're writing unix programs, or if you are writing programs on a unix.

And symbiotically, C was written to write unix.

edit: to note that by 'written' i mean designed and initially implemented.