It is probable that Mormon wanted to make certain that the timing of finding Amalickiah dead was given appropriate attention. In many cultures, the first day of a new year is auspicious, and finding the king dead on that very day would be a bad omen. When that was compounded with finding that the Nephites were ready to give battle, though not attacking, the prudent response was to retreat to one of the fortified cities and assess what should be done.
Mormon suggests that the Lamanites were thwarted in their designs to continue northward. Mormon would not have known what those designs were, but from this time forward he will increase his attention on the lands to the north. In this story, he is only laying those foundations.