Rust is built on LLVM, much like Clang. C integration is relatively easy. I think this is more viable than Go. Unfortunately, Rust is only portable to platforms LLVM supports. While that covers the majority of commonly used hardware today, it is not comprehensive. Also, Rust compile times are pretty bad, especially for optimized builds.
At the end of the day the author gets to write code in a language of their choice to scratch their own itch. End users don't care at all what language a web framework is written in.