It was important for stability in ancient societies to have as few transitions of authority as possible under monarchical government. Every time a king’s throne was vacated, the social structure of the whole community was upset. Generally, the kingship became vacant upon the monarch’s death. It was highly unusual, almost unthinkable, for a king to give up his office voluntarily or for a king to be replaced by popular vote. Regardless, when a king died or left office, all government officials serving under that king were automatically released and replaced by the new king. The new king had the right to replace or keep any government official. It was analogous to what happens in the Church today when a bishop is released. The counselors in the bishopric are generally released with the outgoing bishop. Similarly, when the Relief Society President is released, her counselors are usually released with her. In the ancient world, it was not unusual for a new king to simply reinstall the previous bureaucrats, officers in the military, and other government officials. However, change to monarchical authority was often difficult and took a period of time before stability was resumed in the community. This may provide some explanation as to why leadership was often passed on to a younger son of the king.
There may be another reason why the younger son of a king would assume office—the older sons were often needed as workers. In an agrarian world, the older sons were needed to till the fields and take care of the herds. They typically worked in agriculture or learned to be businessmen or craftsmen. Usually the younger sons—the favored sons—stayed home, studied, learned languages, and became educated and trained. They had a closer relationship with their father. I think we see that with Nephi. Nephi recorded, “Having been born of goodly parents, therefore I was taught somewhat in all the learning of my father.” Nephi was the record keeper. He became the successor to Lehi.
When you look at how succession in the early part of the Book of Mormon went from Jacob to Enos to Jarom and to Omni, only a few generations covered a lot of years. In order for that to make sense, Enos was likely born very late in Jacob’s life; Jarom was likely born very late in Enos’ life. As the king or patriarch of a family aged, he could leave much of the day-to-day dealings to his older sons. The father would have more time to train and school a younger son as successor and give him all of the necessary expertise in politics, linguistics, and economics. A powerful king or patriarch would not have wanted to train his sons too soon, lest they upstage the father. Ancient kings and patriarchs often selected their younger sons as successors.