Mormon has not made it explicit until this point, but everything he has written about the way the Nephite apostasy has occurred in the past should have prepared us for these verses. We learn, explicitly, that one of the aspects of the changes in society under the Gadiantons is the rejection of the teaching of the coming Messiah.
Nephi began with Moses’s leading the children of Israel to freedom, implying that he desired to do the same with the Nephites under Gadianton rule. Now, he uses Moses again, but this time to declare that the prophets of old had preached of the Messiah. For this, he selects the raising of the serpent. The pole with the brazen serpent was raised so that all might see it, and all needed only to look upon it to live. It was a simple thing, but one that many declined to do. Thus, declares Nephi, the understanding of the coming Messiah is taught and available, but many have rejected it.
Nephi then transitions from Moses to the even older founder, Abraham.