Among other things, tooling and vendor libraries. Vendor libraries are often composed of thousands upon thousands of lines of auto-generated C headers and written in some bespoke format. Demonstration code and/or sample drivers are almost invariably provided in C. Of course you _can_ rewrite these in Rust, but if you're an engineer trying to get shit working, you'd first basically have to reinvent the whole wheel just to do bringup.
I don't even want to talk about the state of proprietary vendor tooling...
I don't even want to talk about the state of proprietary vendor tooling...