The Lamanite/Gadianton army is enforcing a type of scorched earth policy. As they beat back the Nephites, they completely destroy the cities that they take. This tactic removes usable land and resources from both armies. It is effective for an attacking army if they can move swiftly enough, of if they have a strong enough line of supply. Apparently, the Lamanites had both.
Chronological: Three hundred and seventy nine years would be 370 A.D. Mormon the general is now about 69 years old.