Mormon presents the basic information about the conversion of the Lamanites, but does not give us the sermons that produced the conversion. This is in contrast to the way he presented Alma’s missionary journey. We can never be certain why Mormon leaves out information, but perhaps the message of Nephi and Lehi had already been covered in other sermons that had already been included.
There is a reason for the absence of the sermons that can be suggested, however. Mormon’s narrative pacing is increasing as he approaches the arrival of the Savior. He has built up the Nephite understanding of religion and the atoning Messiah in earlier parts of his text, but now he is moving towards the New World mission of the atoning Messiah. That event is the spiritual and textual focus of all that Mormon writes, so he does not delay our approach to that event with too many sermons. The explanation of history is sufficient.
In this case, the missionary journey is essential not only because it happened, but because it demonstrates repentance. For Mormon, it should be harder for a Lamanite to believe because of the “wickedness of the traditions of their fathers.” The tradition of the Nephites includes faithfulness, if only they would remember. This mission is therefore symbolic in its presentation more than it is instructive. Mormon’s intent in including it is not for the doctrine, but for the example that it provides. Regardless of where we are, it is possible for the spirit to touch us and return us to our God.