Ammoron still had the advantage of the larger land from which to draw his army. Unlike the Nephites who were battling on two fronts and had a relative small population that had to have its defensive army split into two major defensive sites, the Lamanites had a larger homeland at its back. It is true that their forces were also divided, but with the larger land to draw from, this was much less a disadvantage for them. It appears that Ammoron did not need to the men enough to give up anything significant for them. It was more economical for him to lose men than it was to return fighters to the Nephites.
This suggests that he was receiving reinforcements from the Lamanite homeland along with the supplies that undoubtedly were being delivered. The lack of value of Antiparah is clearly demonstrated in that the Lamanites holding that city simply flee and hand 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.