RH does have an interest in Rust, used in projects such as Stratis. It's just that the Linux dev ecosystem has been very C-reliant for a long time, and a massive amount of binding and other ecosystem work is still happening to make this possible.
*EDIT:* and the reason I mention Rust specifically is that, in these types of lower-level projects, a lot of things can start to get hairy very quickly in higher level languages. Things like some namespace APIs very much wanting to be run on a single thread, or trying to maintain performance when you're intercepting and examining every D-Bus message, or even just when you want your functionality to be in a reusable core library.
Meanwhile RedHat and others don't have infinite resources, and they inherited sudo.
Languages like D even allow you to disable the garbage collector for specific methods, giving you the benefit of GC-less performance and characteristics in critical code paths and the YOLO memory management of GC languages in the wrappers around them.
I guess the answer is "because all the people over at Redhat know C"
Just to throw out some guesses:
1) "because dynamic linking", if you're an OS vendor being able to ship one package to fix a security vulnerability instead of 50 is a big deal
2) you probably want to have a C-compatible API anyway so that it's possible to use the code from other languages, so combined with 1) the benefit of a language like Rust or D would be dulled slightly. Not eliminated, just reduced.
3) Less common architectures POWER and s390x are still relevant platforms in enterprise and while languages like Rust do have some support for them, C compilers for those platforms definitely get more attention.
