Mormon has set the stage by announcing the faithfulness of the Lamanites. Earlier, it would have been unthinkable that a Lamanite would preach the gospel except for the Lamanites converted by Nephi and Lehi, who also mounted missions to the Lamanites. Now, when Yahweh calls a prophet to summon the Nephites to repentance, he chooses a Lamanite.
Not surprisingly, the Nephites reject Samuel. Nephi is living among the Nephites, and his two amazing miracles have only temporarily influenced Nephite repentance. If they were able to harden their hearts after all that Nephi showed them, how much less would they listen to Samuel? As a Lamanite, the Nephites were predisposed to dismiss him. Nevertheless, his righteous presence and rejection stand as a striking contrast in the cultural expectation that the Nephite would be righteous and the Lamanite wicked. Samuel demonstrates the complete reversal of that expectation.