User blog comment:Miskos3/Fairy Tail Chapter 505: Trump Card, Review/@comment-97.104.204.83-20161010124046/@comment-4829046-20161010190810

I can give a possible reason; because Mavis was the only one whom Zeref considered special. Everyone else is irrelevant since Ankhseram seemed to single out Zeref. As you said, others have tried to blur the line between life and death before and those that should have gone for good were somehow able to exist. I am not counting Demons though because they are artificial and thus not part of the governance of Gods. And on that thought, isn't it strange how only some people have the privilege of having 'spiritual' conversations or encounters with the main cast? What about the others? What decides it? Magical power? I think not, because some powerful deceased entities never show up once they are confirmed dead. Ankhseram is just terrible at the job.

Mashima intends it to be a ZerefxMavis link and if another were to appear, it would upset the whole premise of the series that built up Zeref as the primary overarching antagonist whose plot is connected directly to Fairy Tail i.e. Mavis. Heck, Mashima only brought Mavis into the story when it was time for Zeref to appear. Every major thing he built after that was based on that link.