Lamoni’s father is not making only a paternal or personal request but issuing a political order as Lamoni’s overlord. He is forcing Lamoni to choose an allegiance—either to his overlord and father, or to this Nephite. Refusal would be rebellion against his father and all of his father’s allies. He would declare antagonism toward former allies and renounce political, kinship, and economic connections without having replaced them. In an ancient society, such a move could be literally fatal.
Furthermore, it was not just a personal decision but one he was making for his kingdom. Lamoni would be cutting his people off from the economic and social benefits that came from his alliance and reducing their physical security by giving up the shared defensive strength. His city would stand alone, with no new alliances to replace the strength of the one he would be giving up. In the eyes of the world, Lamoni had everything to lose and virtually nothing to gain by refusing this demand of his father and overlord.