Speed and ease.
Supposedly, according to Google, gRPC is 10-11x faster than HTTP/REST. It’s also dramatically less complicated.
Personally, I found protobuf to be a colossal pain in the ass. For me RPC with JSON proved to be about 7.5x faster (plus or minus 0.5x). Since there is overhead to parsing JSON which is less than the overhead of dealing with protobuf the Google approach proved unnecessarily excessive.
HTTP is unidirectional and forces the insanity of round trips. A primitive full duplex socket independently accepts messages from each direction. Process the messages as they come in. It’s just fire and forget. Stupid simple.
With primitive sockets there is no client/server after connection establishment. Everything is a client. Servers just have a service listener and either side can carry both client and server capabilities. When both sides carry both roles that which comes online last is the client.