At one point, the Rust team switched away from the performant Jemalloc allocator to something more widely compatible (the system default). They chose to sacrifice performance in the sake of compatibility/stability. It's still available, but optional.
https://internals.rust-lang.org/t/jemalloc-was-just-removed-...
Insane performance gains, like the ones we see early in Zig, is something that can be easily eaten away by the natural evolution and maturity of a programming language.
Btw, Zig is beating trees with a stick to see what may fall down in this area: https://github.com/ziglang/zig/issues/12484