In case any of his readers forgets who the focus of the story is, Nephi begins by stating that “I, Nephi, and my brethren took our journey.” It is certain that Laman was in charge of this expedition. Laman is the eldest, and it is his rightful position.
That rightful position is emphasized when they consulted with each other on what to do. They cast lots. Casting lots is a method of using chance as a way to decide. Modern readers easily see it as random. However, in the ancient world, the presence of what everyone knew to be chance was a mechanism to invite Jehovah’s intervention.
In this case, they cast lots. Rather than a random outcome, the brothers expected to receive the quiet revelation of Jehovah’s will. The lot fell upon Laman. That would have been confirmation that it was not chance. Although there were four brothers, there was only one eldest. Everyone knew that, by right, this task should fall to the eldest. When it does through the mechanism of casting lots, it is confirmed both that Jehovah wanted Laman to lead, and that Jehovah was behind the selection—precisely because Jehovah chose the eldest.
Other than cultural expectations, why did Jehovah have the lot fall to Laman, if Jehovah knew (as He must) that Laman would fail? God’s foreknowledge does not cause an outcome. The agency we exercise in this life is for our own benefit, and we get that benefit only if we are the ones choosing to exercise our agency. Thus, God provides us opportunities despite our past choices.