ANSI-C (and to a lesser extent C99) is pretty much the only language which you can compile anywhere, forever. For this project portability is the #1 requirement.

If he used, say Rust, then the target system needs a rust compiler, and furthermore, the language is not even close to being as standardized a C is at the moment, i.e. who knows if in the future you will still be able to compile your program that you write on 2023's dialect?