Does json-rpc have a standardized crypto wrapper, though? That seems like the most important reason to use https. Although json-rpc + nacl/sodium might be better in theory...
It looks like they do a good job of tackling performance problems as they see them, even at the architectural level. My guess is they didn't tackle this because it wasn't a problem worth prioritizing compared to the compounded shitshow of a monolithic Rails app.
JSON has advantage of human readability and for that tooling support is required which probably you will loose with RPC. For example I use many tools to inspect requests that won't support it with RPC