600-Year Prophecy

Why wasn't King Benjamin aware of Nephi's 600-year prophecy of the coming of Christ?  The 600-year prophecy of Christ's coming was recorded in the small plates of Nephi and passed down via Jacob's line and not the kingly line which kept the more historical large plates of Nephi.  The small plates were eventually given to King Benjamin but if they had been written in Egyptian script, it is uncertain if Benjamin could have read them. It wasnt until Mosiah 28, that King Mosiah uses the seer stones to translate the Small Plates of Nephi and the Jaredites Plates.