For the same reason people use JSON. It's simpler, and there is a fast parser built into the browser. Even non browser clients have optimized JSON coders whic

For the same reason people use JSON. It's simpler, and there is a fast parser built into the browser. Even non browser clients have optimized JSON coders which is good enough. That said, JSON is still slower and more bloated than Protobuf, and you get Protobuf for free when using gRPC.