Mormon specifically blames the Nephite defeat on their eagerness to take the war to the enemy. This might be a reiteration of the Nephite preference to avoid wars of aggression, as indicated in Alma 43:47: “And again, the Lord has said that: Ye shall defend your families even unto bloodshed. Therefore for this cause were the Nephites contending with the Lamanites, to defend themselves, and their families, and their lands, their country, and their rights, and their religion.”
It may also have had to do with the strength of their defensive position, which was lost, if they left it to attach a stronger army. Perhaps it was both reasons.
Ultimately, Mormon blames the defeats on Nephite wickedness.