Moroni knew that the Saints of the latter days would have the Bible and therefore he left out everything set forth on the 24 gold plates that covered the sacred scriptural history from Adam to the building of the Tower of Babel. It was when the Lord said the Tower of Babel was about to be destroyed and the people scattered all over the earth that the history of the Jaredites actually begins.
Although Moroni says he cannot give us the full account of the Jaredites, at least he will trace their history from the time of the Tower of Babel around 2200 B.C. to the final destruction of the Jaredites, which we believe to be around 250 B.C. This history therefore covers a period of nearly 2000 years.