I read them at the time. The gist was basically "it has been empirically established that async is the right way to do concurrency" - ie, it was because of Node.
It was more "it fits best with our needs as a systems language (overloaded term, yada yada) and we should support a robust concurrency model sooner rather than later". The Rust devs were aware of green threads and the like. It is deeply unfortunate that async is still such a mess, though.
