Could Wireguard have been written in Rust or does it require C for some reason? We've seen CVEs in major libraries (e.g. openssl) and some of the guarantees Rust provides would prevent certain classes of vulnerabilities.
Has the author not learnt from the past or does it actually need to be written in C for some reason?
You don't seem to have spent a single minute researching before posting :/
The original implementation is a kernel module. You can find a bunch of (excellent) talks online by wireguard's author which all have a focus on security aspects and avoiding vulnerabilities. Besides, Go and Rust (3rd pty) implementations exist.
If you want a module to be accepted in to the Linux kernel upstream, then you have to comply with the kernel's rules.
The kernel uses various C macros and GCC extensions. Ensuring that a module written in another language was compatible, even given that it would need to be built outside the normal tree would make its maintenance within the kernel tree impossible.
In terms of being written in C++/Rust/whatever, as an external module that complies with the kernel ABI (not guaranteed between kernel releases), go for it.
But if you want to have your protocol/module in the standard kernel tree, C is your choice.
Has the author not learnt from the past or does it actually need to be written in C for some reason?