The Imperial Scroll was so important, and was the foundation of an Imperial Clan. No matter what, the Imperial Scroll could not possibly be taught to outsiders.
For example, many Imperial Clans had high hopes for Ling Han. In fact, they were even willing to invite Ling Han over and betroth him to a female clansman who wasn't very important. However, Ling Han would only be able to learn the Imperial Technique in this clan, at most, but he would never be able to see the Imperial Scroll in this lifetime.
Even among the Imperial Clans, those who were not important successors were unable to cultivate the Imperial Scroll, and those clansmen that cultivated the Imperial Scroll all placed restrictions in their minds. If outsiders wanted to forcefully steal it, the restrictions would activate and damage the mind.
They would rather have their clansmen die. Even if he was the Imperial Son, he would definitely not teach the Imperial Scroll to outsiders.