User blog comment:Miskos3/Fairy Tail 541: The Magic of Hope, Review/@comment-1248963-20170626143209/@comment-21119641-20170630125216

He says he makes the story as he goes and even admitted that Mavis gained a bigger role in the story because the fans seem to like her.

So, the answer is Fanservice. He'll do what the majority of the fans LIKE for the story, not what he thinks is good for it.