The issue was that this work was initially put on hold for a while. When we revived it, we took the time to listen to feedback from early adopters and published the code out.
Your point on 'hidden development' is well taken. It's not something we (grpc team) are usually comfortable with either, but in this case, it was just an unfortunate circumstances that led to this code being in alpha and hidden, for so long after the protocol was released.
For newer efforts, such as the pure node implementation - https://github.com/grpc/grpc-node , we've chosen to take the approach of developing it from the ground up externally.
