Because Mormon is speaking to a future audience that he knows will have spent some time in apostasy, he cannot be certain that they will understand their heritage. It is perhaps interesting that LDS custom has labeled all Native Americans Lamanites. Mormon, however, addresses them as a “remnant of this people.” Mormon is treating his future audience as future Nephites, not Lamanites.
For Mormon it is probable that he understood the politico-religious meaning of the term, and that while the ultimate recipients of his work would be termed Lamanites for their position relative to the gospel, they were still, in their spiritual bloodlines, Nephites, and more importantly, of the house Israel. Mormon begins with this statement because it is the foundation of the blessings that will come to this line. There are promises made by God to the house of Israel, and Mormon wants his future readers to understand that they are potential participants in those blessings.