Nephi doesn't explain much of what goes on during those 8 years. Why did it take them so long? They traveled for a period then stopped and set up their tents. This was the way of travel in the desert, but it doesn't take 8 years to travel that far. They traveled as directed by the Liahona but only when it was working. It must not have been working because the Lord was punishing the rebellious in the family—Laman, Lemuel, the sons of Ishmael and their wives for murmuring. Speaking of Lehi's family, Alma later recorded, 'They were slothful, and forgot to exercise their faith and diligence and then those marvelous works (ie. the Liahona) ceased, and they did not progress in their journey; therefore, they tarried in the wilderness, or did not travel a direct course, and were afflicted with hunger and thirst, because of their transgressions…for as our fathers were slothful to give heed to this compass (now these things were temporal) they did not prosper; even so it is with things which are spiritual' (Alma 37:41-43).
This is exactly what happened to the children of Israel. The Lord was prepared to bring them into the land of their inheritance soon after they departed from Egypt. However, the people were so wicked and faithless in their attitude about battling the inhabitants of the land, that the Lord made that entire generation wait (for 40 years). Joshua and Caleb were the only adults of that generation that did not die before the children of Israel were allowed to inhabit the land (Num 26:65). Thus the forty years of wandering in the wilderness were the result of the idolatry, faithlessness, and murmuring of the children of Israel.