8 And these things have I written, which are a lesser part of the things which he taught the people; and I have written them to the intent that they may be brought again unto this people, from the Gentiles, according to the words which Jesus hath spoken.The account contains the lesser part the Lord wanted to reveal. Why? To "try their faith" and if they decide to believe these things the greater things will be revealed. If they do not than it will be to their condemnation. The Lord tells us concerning the Book of Mormon.
9 And when they shall have received this, which is expedient that they should have first, to try their faith, and if it shall so be that they shall believe these things then shall the greater things be made manifest unto them.
10 And if it so be that they will not believe these things, then shall the greater things be withheld from them, unto their condemnation. (3 Nephi 26:8-10)
26 Verily, verily, I say unto you, that there are records which contain much of my gospel, which have been kept back because of the wickedness of the people;They contain "much of my gospel". Not all of it, just much of it. Thats because we still have not had the knowledge that the ancients used to know revealed to us. They had great knowledge of the constellations which testify of God. For God gave signs in the heavens to testify of His ongoing work and His ordinances. They remain pure. Men cannot change them. They testified of the Lord's birth. And testify of all things from the beginning and the end. Moses saw these things and learned of them in the book of Moses. Despite having only much of the gospel why do we feel we have more than all those who came before? Why say we are righteous when we are still under condemnation? Why say more will be given when we have rejected the gifts and knowledge the Lord has given us through Joseph?
There is still much to be revealed. We shouldnt think we know a lot when we really know very little.