I don't know the intention of the Rust team, but maybe it's because of the difference in release cycles: if a bug is found in an unstable feature today, it can

I don't know the intention of the Rust team, but maybe it's because of the difference in release cycles: if a bug is found in an unstable feature today, it can be fixed by tomorrow's nightly build, but it won't make it into stable until at least February. Presumably, the maintainers don't want to release code that is in flux and under active development under the same 6-week release cycle that they release stable, well-tested code.