There are multiple kernels written in Rust already. Writing another one wouldn't be interesting.

The point of R4L is that people want to write drivers for Linux in Rust. The corporate sponsors that are involved also are interested in writing drivers for Linux in Rust. Sure, Google could rebase Android on top of RedoxOS or Fuschia and Red Hat could spend a decade writing Linux Subsystem for RedoxOS, but neither want to do those things. They want to write drivers, for Linux, in Rust.

Telling them to write a new kernel is a bit like telling them they should go write a new package manager. It's a completely different thing from what they actually care about.