Question:

Why Mormon's letters and discourse on Faith, Hope and Charity probably took place after the battle of Cumorah. 

Answer:

Among other reason, presented in the book, Moroni probably put his father's letters and discourse in his own book, because his father's record was of the Nephite nation, and not the Nephites that had gone to the North countries, and were therefore not part of his record. These were the people Moroni ministered to.