I enjoyed my brief time with gRPC and protobuf, when I was trying them out, and I'd happily use them again, but I think it's obvious why JSON and REST are more dominant.

For one, gRPC works great for simple server/client apps, but it makes them VERY tightly coupled. If your product is an API itself, open for public use, tight coupling is a hindrance more than a benefit.

And more to the point... have you ever tried to DEBUG gRPC traffic? The efficiency of gRPC is top notch, t optimizes for serialization, deserialization, and bandwidth, all at once. But JSON is human readable, and when you throw in HTTP compression, the efficiency is close enough to protobuf to serve the vast majority of scenarios. JSON is just far more practical.