This simplification by Jacob is consistently adhered to in the Book of Mormon. What should be remembered is that this is just that—a simplification. The Book of Mormon was written for us, in plainness, to our understanding. The details of peoples, customs, cities, wars, etc. could not be fairly dealt with in such a small record. As Mormon admitted, I cannot write the hundredth part of the things of my people (W of M 1:5). Therefore, it makes it easier for us to understand the story if the “good guys” are called “Nephites” and the “bad guys” are called “Lamanites.”
Daniel H. Ludlow explains these terms as follows:
"In general, the terms ‘Nephites’ and ‘Lamanites’ are used with the same meaning for the first 500 years of Nephite history. The term Nephites refers to all those who followed after Nephi and to their descendants. The term Lamanites refers to those who followed after Laman and to their descendants. However, it is mentioned later in the Book of Mormon that there were no ‘ites’ of any kind during the 200-year Golden Age immediately after the appearance of the resurrected Jesus Christ. (See 4 Nephi 1:17)
"After this 200-year period of righteousness, the terms ‘Lamanites’ and ‘Nephites’ are used again, but with somewhat different meanings than those used earlier in the Book of Mormon…
"…In A.D. 231 ’there arose a people who were called the Nephites, and they were true believers in Christ ... therefore the true believers in Christ, and the true worshipers of Christ ... were called Nephites. ... And it came to pass that they who rejected the gospel were called Lamanites.’ (4 Nephi 1:36-38)
“The terms Nephites and Lamanites for the remainder of the Book of Mormon are determined by this division, which had taken place by A.D. 231. In other words, the Lamanites of the last two hundred years of Book of Mormon history are descendants of those who revolted against the true church of Christ between about A.D. 194 and 231.” (Daniel Ludlow, A Companion to Your Study of the Book of Mormon, p.156, 296)