Well, it wasn't always "secret". The cult is the "open secret" that is referenced in Eyes Wide Shut, both in the script, and the title. Rome was quite public about its rape of children and demonic worship of make-believe creatures, and even darker things more real than a Regular Joe could ever imagine. It was in the open back then.

I think something about the way humans are just rejects that shit though, so it's become the big elephant in the room, so to speak.