Messagepack is a serialization format, not a transport. It is an alternative to protobuf or json. Mostly people use HTTP because there are so many services and

Messagepack is a serialization format, not a transport. It is an alternative to protobuf or json. Mostly people use HTTP because there are so many services and components that already support it (load balancers, proxies, sidecars etc), it has well-supported options for authentication, conveys metadata seamlessly through multiple layers, and its overhead isn't an issue for the kind of payloads they are sending. Plus, you know, curl.