When secrets are written down on paper, then it's hard for people to share them without buying more paper from you. Sure, people can copy the paper and publish it for free, but convincing them not to is still a scalable solution. Printing takes work. At least, it did when copyright was invented.

When secrets are published on the internet, anyone can publish them again for free. Worldwide. Instantly. Literal billions of copies if they feel like it. There's no holding that back. It's not remotely scalable.