To unlock the new Cataclysm difficulty, all Lords must be defeated, even if this has been done previously.
We've made this choice as Cataclysm isn't supposed to be a part of the normal progression, and a lot has changed with regard to balancing and mechanics. We implemented this as a skill check to ensure those who participate in Cataclysm are ready, to protect both those who are playing Cataclysm, and those who aspire to do it.