What's wrong with HTTP? If you're going to build an RPC protocol, you are going to need encryption, stream multiplexing, a way to distinguish RPC methods, support for extensions (e.g. auth), and compression.
Add the fact that high performance HTTP servers and clients exist in most languages, and building an RPC protocol on top of HTTP sounds pretty attractive. No wonder gRPC did exactly that.
Tons of round trip calls potentially in most RPC-style systems are one start, which Cap'n Proto addresses with its unique style of avoiding round-trips for many network transactions which can drastically reduce network chatter. Security is addressed simultaneously (but not necessarily sufficiently for many people or regulatory bodies) in Cap'n Proto at least via use of capabilities instead of building it into transport layers.
Add the fact that high performance HTTP servers and clients exist in most languages, and building an RPC protocol on top of HTTP sounds pretty attractive. No wonder gRPC did exactly that.