User blog comment:IamJakuhoRaikoben/Chapter 357: The Nine Demon Gates, Review./@comment-5580383-20131025195248/@comment-7418318-20131026082108

Either that or they just kept quiet because they trusted the current council would reveal or use that knowledge when the time was right.

Or maybe they thought that Tartaros being run by Demons was just a rumor and thus didn't think they'd need anti-demon magic (or whatever) against them.