As far as I can tell C was pretty much the only language available to build Linux with at the time. From purely practical considerations never mind any subjective preferences and emotional attachments programmers have for their favourite languages.
Also it makes a lot of sense to me that having selected a language for such a project there would have to be massive advantages in anything else to even consider it. Having a polyglot system is going to be a lot harder to hold together. There have been no languages available with such massive advantages.
Enter Rust... For the first time, since Ada perhaps, a language arrives that does offer new features in terms of program correctness, which are very valuable in projects like an OS kernel.