User blog comment:FairyRaven/Fairy Tail Chapter 400: Wings of Hope, Review and Commentary/@comment-1359322-20140913124543/@comment-4489126-20140913165750

I think it's because Acnologia is not a Dragon, nor a Slayer in a way. So Curse my be the only thing that can actually harm it. And since E.N.D. is Zeref's strongest Demon~