In terms of compilation of programs Go is far, far easier than Rust. For Rust to compile a random Rust program on the internet one almost always has to have the absolutely latest out of repo compiler toolchain from curl rustup.whatever | sh. The normal 4 year release cycle is incompatible with rust development. For commercial use cases this doesn't matter. But for open source like the tor project it does.

That said, since they use Firefox this bridge has already been burned.