The withdrawal of the spirit of the Lord from the Nephites comes because they have abandoned the principles of the gospel and adopted those of the world. However, the statement of that withdrawal serves Mormon’s editorial purpose in setting up these Nephites for destruction. While they were righteous they were under Lehi’s foundational promise. Now that they have moved away from the gospel, they are no longer under God’s protection, and they will be destroyed, according to the penalty associated with Lehi’s promise.