Mormon seems to suggest that there was a general prosperity in which both the Nephites and the Lamanites shared. That prosperity led both people to pride, and, therefore, both people to wickedness.
Mormon is preparing his readers for the final demise of the Nephites, and contrasts the previous righteous state of both Lamanites and Nephites with the nearly universal separation from gospel principles in the land. Even though we still have the labels of Lamanite and Nephite, they can no longer be used to suggest righteousness versus wickedness. They were both “exceedingly wicked, one like unto another.”