What strikes me as I reread these chapters of Ether is an absence of direct references to Jesus Christ, as compared to all other parts of the Book of Mormon. Moroni repeatedly uses generic terms such as The Lord and refers to the need to repent, but he does not discuss core doctrines such as the atonement or salvation.
We know from the great theopany of the Brother of Jared that at least some of the early Jaredites knew about Christ thousands of years before he came. But it isn’t clear whether memory of Christ got lost over time and became replace with more generic theological ideals.
It seems to me likely that one reason the people were so quick to forget God and to become wicked was that they had forgotten Christ. Without good scriptural records of the teachings of the Hebrew Prophets, they had fallen away over time.