I am pretty sure Java's historical use of green threads was an issue of portability and not performance. They used traditional threading primitives. Java's decision to use green threads may have actually made a lot of sense in an era when consumer computers typically only had one physical thread to begin with.

Go on the other hand is based on CSP principles and a threading model that looks like actors. The threading model is deeply ingrained in the language, and it is designed around it. Goroutines are very cheap in Go, and the scheduler is fairly effective because it knows what threads to wake when - it's not blind. Goroutines are scheduled across n threads, not just a single OS thread, so they can take good advantage of multicore or multi CPU systems. This design does hurt C interoperability a bit, but imo it's greatly worth it.

Go is not the only language that works this way. I believe its concurrency model was inspired a lot by Erlang with its 'processes' model.