I can think a few reasons:

- Cranelift applies less optimizations in exchange for faster compilation times, because it was developed to compile WASM (wasmtime), but turns out that is good enough for Rust debug builds.

- Cranelift does not support the wide range of platforms (AFAIK just X86_64 and some ARM targets)

So it isn't just a matter of "they would use Rust instead".

There is a whole ecosystem of contributions across the globe and the lingua franca used by those contributors.