That's not a property of the language, it's just an optimization they made in line with the "Release a minimum viable product, then improve it" strategy they inherited from Mozilla.

Just look at how long it took LLVM to catch up to GCC. Writing a competitive optimizer pipeline is hard work.

"No room for a language above assembly and below Rust" is a design goal for what it's reasonable to build with it.

If anything, using LLVM for the default backend is completely on-brand with how much priority Rust puts on being suitable for partially or incrementally rewriting C codebases.

(Sorry for the slow reply. The tab got buried.)