"Now this he spake because of the stiffneckedness of Laman and Lemuel; for behold they did murmur in many things against their father, because he was a visionary man, and had led them out of the land of Jerusalem, to leave the land of their inheritance, and their gold, and their silver, and their precious things, to perish in the wilderness. And this they said he had done because of the foolish imaginations of his heart."
Lehi had to speak to Laman and Lemuel about these things because of the "Stiffneckedness" meaning that they would not accept the word of God. The did not believe in all Lehi had spoken.
By murmuring they complained against their father and God by saying that Lehi, in essence, was a fool to leave their home and their gold and silver. They believed that he had led them to die in the wilderness because of imaginations.
Because Laman and Lemuel spoke against their father and refused to believe his words they were unable to understand the things of the Lord. Because of their wickedness and stiffnecks they were covered by a spiritual darkness making it impossible for them to receive and understanding of the ways of God. This is a way that God protects the things that are sacred. He will not allow His most sacred truths to be sullied by the evil intentions of people.