I’ve always thought the Romulans weren’t just the Vulcans who rejected Surak’s teachings, but also any who didn’t have the physical ability to follow them.

Originally it was just based on Romulans not expressing any psychic abilities, but Picard also established that sharing personal details publicly as a major taboo. That would track with them being a mix of former enemies who are concerned with suppressing the rivalries that lead to nuclear war.