Fortunately, having Ocean Master be the king of Atlantis isn’t unprecedented.
Years later, Arthur Curry found Atlantis for the first time (he’d been raised on the surface by his human father, Thomas Curry), but despite being older than Orm and thus entitled to become the new king of Atlantis, he let Orm stay king.
In the New 52, Orm ascended to the throne after his father is killed and his mother mysteriously died.
It was only after the Throne of Atlantis storyline, where Orm tried to invade the surface, that Aquaman overthrew his brother and accepted the responsibility of ruling Atlantis.