Secret isn't a great word for something you desperately want to share but only selectively.

Secret societies want their knowledge to be passed on and spread, but only to people who they feel are ready and willing to commit to a part in the safekeeping of the knowledge.

Filtering by inferential distance like that is a good way to save yourself wasted time. If someone who can't add asks me to teach them calculus... Worldview has a similar problem. I can't easily sell crypto anarchy to someone who believes the government is good. The inferential distance is too far, I have to change their entire worldview before they can see the value.

Long way of saying, lots of knowledge is effectively secret to many people because they haven't laid the groundwork to be able to learn it. Our willingness to share can never change that. Like the thinking version of asking someone who hasn't climbed the first stair to step onto the 15th stair.

People do want in on a secret, perhaps the imperfect word is marketing.