Mormon notes:
And it came to pass that we did march forth before the Lamanites. And I, Mormon, wrote an epistle unto the king of the Lamanites, and desired of him that he would grant unto us that we might gather together our people unto the land of Cumorah, by a hill which was called Cumorah, and there we could give them battle. And it came to pass that the king of the Lamanites did grant unto me the thing which I desired. (Mormon 6:1-3)
According to John Sorenson, a valid question is, Why didn't the Nephites continue retreating farther and farther north and so escape the Lamanites altogether? [rather than gather at one location for a final battle] In the first place, we must realize that rarely if ever is there any decent land that does not already contain a sizeable population, so the Nephites would have had to dispossess other people first. Besides, moving farther on, the Nephites would have entered ecologically new territory, and the prospects would be slim that they could successfully feed their numbers in a new environment with no time to learn how to exploit the land. Assuming a Mesoamerican setting, farther north of the Nephites' position lay another military threat. Beyond the big swamps [of present day Veracruz] they would come nearer and nearer to the territory of Teotihuacan proper, the powerful state allied culturally if not militarily with the Lamanites (i.e. Kaminaljuyu). The Teotihuacan domain of control apparently did not extend quite as far as the Tuxtlas (land of Cumorah) by A.D. 380, but any move farther north by Mormon's people would have encountered this great power, standing in the wings but uninvolved directly in the present conflict. Yet the real key to the Nephites' standing fast could simply be that the lands they were defending were their own already; they felt they had a right to them and were motivated to defend them if at all possible. . . So, caught between the millstones of Lamanite powers and Teotihuacan itself, the Nephites willingly defended their shrunken core of lands because they had to. [John L. Sorenson, An Ancient American Setting for the Book of Mormon, p. 348]