User blog comment:Miskos3/Fairy Tail 511: The Hell of Starvation, Review/@comment-95.91.253.211-20161125202542/@comment-7418318-20161126192739

With how the land was rearranged, it is possible that Irene put him in a deep cave and Acnologia has to dig his way out first. That might account for the delay.

Hmm... in an extreme case Irene might even have used all the "excess" landmass by which the peninsular was shrunk to create a prison around Acnologia to delay him as long as possible.