Sagawa was intentionally holding back knowledge. He even admitted it himself. Are you trying to suggest that it is a cultural thing for Asians to share their knowledge like that? They are notorious for keeping things secret from people! Especially when you take into account the "koryu" or "mura (village)" mentality of Japanese...a old school dude like Sagawa will not JUST share with anyone.
Not only that, but the prevailing "old way" of teaching (which persists today) in many Asian cultures was to give specific things to people on a personal basis, tailoring what the student was taught to his individual traits and nature. In a roomful of people, each person would be given something to work on, and it might be quite different than what the student standing next to him would be given. As a result, individuals got certain things and developed in a particular direction that might be good for them, but they did not receive or learn the entire system. In that environment, typically one person was selected to learn the entire art and carry the body of knowledge into the next generation; the rest learned varying amounts of the system, but not the innermost core (the "family jewels").