Ammoron still had the advantage of holding at least three cities within the borders of the land of Zarahemla. Unlike the Nephites who had a relative small population and had to split their army between two fronts, the Lamanites had their larger homeland at their back. True, their forces were also divided between two fronts; but with the larger land to draw from, this was much less of a disadvantage for them.
Ammoron apparently did not need the imprisoned soldiers enough to trade anything significant for them. It was more economical for him to lose men than to return fighters to the Nephites. This fact suggests that he was receiving reinforcements from the Lamanite homeland, and almost certainly he was also being supplied regularly. As a result, the Lamanites holding Antiparah simply fled, abandoning the city to the Nephites without a fight. Ammoron had already decided that it was expendable, and he did not waste any of his troop strength defending a city he did not need to hold.