The issue is with creating more work for others. Supporting a multi-language toolchain and build environment is a lot more work than a single language one. The R4L folks have made it their mission to shoehorn Rust into the kernel and they’ve explicitly stated that they will not avoid making more work for others. This has upset some longterm maintainers who did not sign up for additional workload.

Linus Torvalds has been accused of many things but he has always been loyal to his best maintainers. That’s been a big key to his success.