Why do so many Christians not know that Lilith was the first woman? And why does no one know of the lore that says that she was created from the earth as was Adam as opposed to being created from his rib?
Because that particular fairytale isn't part of their religion, but was made up long after the Bible was written? Seriously, this question is like asking why most Christians don't believe in Baha'i or Qabbalah.
Outside of some feminist cults, I doubt anyone who didn't play Vampire: The Masquerade has ever heard of Lilith.