> looking to zig as a C replacement rather than Rust
Rust isn't a "replacement for C", but an addition to it. It's a tool that Torvalds et. al. has recognised the value of and thus it's been allowed in the kernel. The majority of the kernel code will still be written in C.
I'm no kernel maintainer, but I can speculate that two of the main reasons for Rust over Zig are the compile time guarantees that the language provides being better as well as the rate of adoption. There is a lot of work done by many leading companies in the industry to provide Rust native code or maintained Rust bindings for their APIs. Windows devs are re-writing parts of _their_ kernel in Rust. There's a "movement" going on that has been going on for a while. I only hope it doesn't stop.
Maybe the maintainers feel like Zig doesn't give them enough over C to be worth the change? Many of them are still opposed to Rust as well.