Why does gRPC use HTTP/2 as transport and not TCP or raw unix sockets?

Question

Programming

gRPC is a newfangled framework from google for communication in microservice architecture. It uses protobuff for serialization as the HTTP/2 transport. The main question is in the title. If you transport HTTP, then why protobuf, what is wrong with JSON, which easily digests browser JS within simple AJAX or websockets (browser support is not implemented for gRPC). If protobuff, then why transport such a high level, why not TCP or sockets.
Along the way

1. If anyone has used it please let me know how you feel. Speed, resource intensity, ease of development?
   
2. What is better/worse than ApacheThreaft, zeroMQ?
   
3. Why are the authors pushing for mobile, why is gRPC so good for mobile development?
   

Answer 1

On the main page it says
HTTP/2
Building on the HTTP/2 standard brings many capabilities such as bidirectional streaming, flow control, header compression, multiplexing requests over a single TCP connection and more.
These features save battery life and data usage on mobile devices while speeding up services and web applications running in the cloud.