All I can really say is that the cryptonote protocol was probably designed to focus on privacy within the blockchain. Intrablockchain privacy. Anything out-of band was not considered within scope, maybe. I'm just speculating because who knows what the cryptonote developers were thinking. The Monero core team is working to advance what cryptonote started, and subaddresses are a step in the right direction.
https://github.com/b-g-goodell/research-lab/blob/master/publ...
All I can really say is that the cryptonote protocol was probably designed to focus on privacy within the blockchain. Intrablockchain privacy. Anything out-of band was not considered within scope, maybe. I'm just speculating because who knows what the cryptonote developers were thinking. The Monero core team is working to advance what cryptonote started, and subaddresses are a step in the right direction.