Well for one, I don’t want to add another language to my tool chain. Many languages can compile C directly. For instance in Swift or Go you can add C source files directly to your project and have them compile as part of your Swift or Go build. You can’t do that with Rust or C++.
C is the lingua franca of the software development world.
That being said, it’s too easy to do something wrong in C. The desire to use Rust isn’t because C is stale, rather it’s too hard to write C correctly.