That's only true on Linux and other systems where the kernel has been written in C. On some platforms the kernel provides no backwards compatibility guarantees and that means backwards compatibility is handled by libc. This means C is enshrined into the platform and often the path of least resistance.

There was also a pure Rust Wayland binding that failed because Wayland assumes that you are using C semantics which made it hard to provide a safe interface for Rust that was also convenient.