Moses descended from the mountain with scripture. The brother of Jared descended the mountain with the command to write scripture. A major difference was that Moses’s scriptures were to be given immediately. Those that the brother of Jared were to write “were forbidden to come unto the children of men until after that he should be lifted up upon the cross; and for this cause did king Mosiah keep them, that they should not come into the word until after Christ should show himself unto his people.”
Moroni was writing after Christ had shown himself unto his people. The command that this part of the record should be kept until after Christ had arisen explains why the description of what Mosiah read to his people covered only the beginning and did not even allude to the appearance of the Savior to the brother of Jared (see Mosiah 28:17–18).