Its a world where the demon lord is a constant, the mark just chooses the obvious candidate. The Demon Lord just happens to be the secular leader of the Demon Race. As a result the Demon Lord candidates pretty much fight until they decide who's on top. That spills over into the human continent where they would show their worthiness through destruction. The Hero is pretty much the only person who can end this, but humanity isn't completely defenseless. Its just that the Demon Lord is just that powerful, so there's a reason the candidates are candidates.