It might have to do with the overhead of calling into C/C++. Since a project like deno will have to do a lot of interfacing with V8, a language like Rust wouldn

It might have to do with the overhead of calling into C/C++. Since a project like deno will have to do a lot of interfacing with V8, a language like Rust wouldn't have the overhead of cgo when dealing with the embedded VM. Also, while go routines are one of the strengths of go, they don't necessarily play well with concurrency constructs in other languages. Since a project like deno will probably be keeping Node's single-threaded, evented model of concurrency and using libuv (also a C interface), that basically means avoiding go routines. To your point, though, you'd still use basically the same language on the server and client, since deno runs typescript and typescript can compile to run in a browser. The Go side would take the place of Node's native modules, which currently have a C++ interface.