Because borrow checking > GC. When you have a GC, you need several times more memory to run the same program with the same performance as without -- and usually, you have to say goodbye to any sort of determinism in execution time, as well.

What Rust brings to the table is guaranteed memory safety without GC, and all memory is released in strictly deterministic time.

So for an OS, it's much better to bring the good bits of Erlang to Rust.