Is this surprising? I don't see an ecosystem of languages targeting the Go runtime, in contrast to the JVM, Javascript, Erlang, MS CLR...

I don't think this is necessarily a matter of popularity or maturity. I don't know of languages that compile to Python bytecode, either. In Python's case I suspect the VM is so optimized for Python that it makes an inflexible target for other languages: I don't know enough about this subject to say whether that's the case for Go as well.