When the Nephites are driven out of almost all of their lands, they are concentrated in the land Bountiful. That land provided a better defensive position because it was narrower, and therefore an enemy would be less able to flank the Nephite positions to get around behind them. It was a defensible line both in being narrower, and being protected by the west sea and the sea to the east.
Moronihah therefore concentrates on improving the defenses along that line, perhaps with an eye to moving his own population more to the north while keeping a tight grip on the new southern border.