it's more niche than C, has less competency available, works very differently to C, and requires a whole new toolchain to be added to the already massive kernel compilation process. for it to be plain sailing adding it to the kernel some of the worlds' foremost domain experts on operating systems would have to re-learn basically everything.

~~also since rust is just coming up on 15 years of existence without a 1.0 release, there's no way to ensure that the code written today will be considered well-formed by the time 1.0 hits.~~