Because writing Rust backend is needlessly complex for majority of projects.
Also, writing JavaScript for the backend is needlessly underperforming for anything with any load.
Node dependencies are fine, add an npmrc file to have it default to exact versioning and you solve 90% of common day to day problems. It's not ideal, but nor is cargo's mystery meat approach to importing optional features from packages.
Maybe leave JavaScript on the browser, where it belongs.