We are fortunate to have in the Book of Mormon—and this is something that we do not have in the Bible—a decade by decade walk with groups like Benjamin and his people who were making the transition from following the Law of Moses up to the time when they were prepared and ready to receive Christ. In this history, we see the Lord sending holy prophets and guiding them step by step (3:13). They were given basic rules along with sublime promises under the law of Moses (3:14), as well as types and shadows (3:15) for those who would understand and not look beyond the mark.
The testimony of Christ is not a light switch that just gets turned on and off. There is a process of transition, and steps are taken to prepare these people. The Lord was heavily invested in this project. He had promised many things to Lehi and Nephi. They had done everything that he had asked them to do. Yet, at several junctures in Nephite history, it becomes perilously close to not working. The Nephites could vanish as quickly as had the Jaredites, and so, at this point, King Benjamin is giving these people a deeper understanding of the Atonement and a deeper understanding of their relationship with God so that they can be prepared and ready.
Terrence L. Szink and John W. Welch, "King Benjamin’s Speech in the Context of Ancient Israelite Festivals," in King Benjamin’s Speech: "That Ye May Learn Wisdom," ed. John W. Welch and Stephen D. Ricks (Provo, UT: FARMS, 1998), 174–183.