User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-4547267-20140716020122/@comment-1359322-20140716020927

No it's probably so that they can control better. If a Maou sits at their positions for too long, people might end up treating them as the real one rather than a symbol, so by having the Maou change time to time, people will not view that current Maou as a real one.