It covers a different use case. JSON, MessagePack and CBOR fall into the "schameless" format which mandate a common but useful enough data model (CBOR is novel in that this data model can be somehow extensible too). Cap'n Proto and Protobuf fall into the "schametic" or "schemaful" format where you always need the correct schema to encode and decode the thing, but is possibly more efficient in encoded size and general performance.