Helaman 4:19

therefore they did abandon their design to obtain the remainder of their lands, for so numerous was the Lamanites that it became impossible for the Nephites to obtain more power over them; therefore Moronihah did employ all his armies in maintaining those parts which he had taken.

So they gave up trying to get the rest of their lands back. The Lamanites had so many people that the Nephites could not beat them. So Moronihah used all his armies to keep the lands he had taken back.

So they gave up trying to reclaim the rest of their territory. The Lamanites were too numerous—it was impossible for the Nephites to gain any more ground. Moronihah focused all his forces on defending what they’d already taken back.