Why would his sons decline to accept the kingdom? Certainly they would have all been trained from birth in their responsibilities to the kingdom. The one intervening circumstance is their spiritual transformation, which gave them a new priority so intense that they would abandon their inheritance in favor of preaching the gospel. Because any remaining son would have been hard pressed to find a reason to decline the throne, it reinforces the assumption that these four were Mosiah’s only sons. It is quite clear from the dynastic histories of virtually all known kingships that competence would be rare as the decisive factor. Their decision to decline the kingship was a direct outcome of their complete change of heart.
Nevertheless, something else is going on that Mormon does not tell us. Even if none of Mosiah’s sons would accept the throne, royal lines are seldom so narrowly defined. In Mesoamerica such occasions occurred not infrequently among the later Aztecs; in those cases, the throne would pass to the son of a brother, or perhaps to a grandson. Thus, Mosiah’s sons had not only declined, but there must have been no other possible heir for this coming change in government, or else Mormon is skipping over some of the details and simply going to the results: the transition from kings to judges among the Nephites.