Mormon summarizes Nephi’s and Lehi’s astonishing success among the Lamanites but does not quote these life-changing sermons, unlike his technique in narrating Alma’s missionary journey. I suggest that Mormon omits them because his narrative pace increases as he approaches the climactic event of the Messiah’s arrival. That event is Mormon’s spiritual and textual focus, so he does not linger over details.
Given Mormon’s innate biases, it should be harder for a Lamanite to believe because of the “wickedness of the traditions of their fathers.” The Nephite tradition, in contrast, is one of faithfulness, if only they would remember. This mission is therefore more symbolic than instructive as Mormon narrates it. It is a historical/ literary example that, if we will remember our God, we can return to our covenant relationship with him.