User blog comment:FairyRaven/Fairy Tail Chapter 301: The Dragon King, Review and Commentary/@comment-4933386-20120929060029/@comment-5268982-20120929175901

What I think is, Acnologia only appears at the end of an era. He first appeared when he annihilated most (all?) the Dragons, over 400 years ago, creating an era of human domination. He then appeared on Tenrou Island, after Zeref said that a new era was dawning (although personally I've noticed no difference yet...except if it has to do with the Exceeds coming to Earth somehow?).