In my urban fantasy world, magic users exist alongside regular people without them knowing. The world's history is the mostly same as ours except that magic secretly existed the entire time. Real life superstitions and beliefs about magic exist and are mostly wrong.
Witchcraft is based on potions and rituals. Few people who attempt magic are actually successful because so little is known about it. Most modern day people didn't believe in magic until now. At the start of the story magic is known to be true by everyone.
What would stop some magic users from telling everyone the secrets of the craft earlier on in history.