Features cost, even optional features. No programming language can or should try to be everything to everyone: to attempt it is to serve no one very well at all, because you split the focus of the language development between too many use cases and you complicate the language so thoroughly that onboarding a new programmer becomes an exercise in re-education about which subsets are acceptable in the given organization.
Rust's niche is safely managing memory without a GC. If Rust adds a GC, even an optional one, it will have lost its soul.