It's called dogfooding -- writing tools for the language in the language. Not doing so here, where the result is "best thing to happen to the ecosystem in a decade", is a tacit admission that Python isn't up for the task of writing best-in-class Python tooling (the use of Rust wasn't incidental). Having seen uv, people will probably start writing more Python-ecosystem projects in Rust.

Which is fine, Python is not for everything.