The origins of the matter can be traced back to the previous High Priestess, Hela's mother.
The status of the High Priestess was second only to the King. She held control over all the temples of Atlantis and was responsible for conveying the will of the gods.
Such a vital position was naturally something the royal family wanted to keep firmly in their own hands. Therefore, Atlantan married the previous High Priestess.
Chapter 1607