Mainly ABI: Data structures (structs), strings, types in C are fairly well defined (by each ABI), this enables other languages to talk to code written in C.

Rust data structures are not defined at all (Unless I'm missing some). For example you can't create a rust string in python and pass it to rust code. In order to pass Rust data to and from another language, you have to create bindings and convert between C strings, arrays, etc + deal with overhead

Zig is the only language I've seen that comes close to matching C in this area. Once it gets a stable release I think many more companies will adopt it.