Mormon catalogues the particular sins of the Nephites, and they are the same sins against which we saw Benjamin preaching near the beginning of the Nephite times in the land of Zarahemla. As we saw at that time, each of these sins is directly related to the acceptance of the outside cultural pressures surrounding the Nephite nation. Mormon is telling us that the fall of the Nephites was their acceptance of the values of the outside world rather than those of God. We can easily concur with Mormon’s assessment, as we have seen these pressures developing in the Nephite polity with increasing frequency since the end of the Book of Alma. Mormon has highlighted those causes for us all along, and simply creates a synopsis of them at this point.
Note that the last of the great sins Mormon lists is deserting to the Lamanites. This has been the direction of social movement all along, and now is increasing so rapidly that there are Nephites who no longer are willing to wait to change their own land, and they leave to the Lamanites to enjoy the benefits of life that they appear to envy in the Lamanites.