The Lamanites apparently attacked at the end of the growing season, when the crops were still in the field and, hence, trampled by the armies. The effect is that the Nephites suffered from the loss of their flocks and herds and grain (Alma 4:2).
Perhaps the Lamanites did not expect prolonged action. If Sorenson is correct that the Amlicites colluded with the Lamanites, then they would expect a quick victory—hence, less need to carry provisions. The Lamanites expected to eat Nephite crops.