Mormon lets us know that he is not including many things that Samuel prophesied. That missing material might have helped us make the transition for the four-hundred-year cycle prophecy that spoke of destructions that would come at the end, but also which were to be fulfilled very soon, near the beginning of that prophesied cycle.
Now, we shift from destruction to salvation. The Nephites have long preached and looked forward to the day when God would come to redeem the people. Just as the four-hundred-year prophecy was particularly relevant in a Mesoamerican setting, so too was a period of five years. Samuel the Lamanite gives us both, and if the Lamanites were more closely tied to the Maya cultures of the time, it is extremely fitting that he should provide prophesies that come in those two significant sets of years.