Many things the ancient prophets were told became lost and Jacob wants the reader to know why. He said the Jews who had custody of the records were often a stiffnecked people. And of course so were our ancestors, the Ephraimites! They despised the words of plainness which God had revealed and killed the prophets. Then they went running after mysteries that they could not understand.
As a result of this, their minds suffered from lack of understanding or blindness, and their blindness came about because they were constantly looking beyond the mark. Now the true mark is what God has revealed. It is simple, and it is beautiful, but it did not satisfy them. So they lost the plain, simple truth and replaced it with mysteries that they couldn't understand. This then came about, not by God's will, but because they desired it. Therefore the Lord had allowed it to come to pass that they might stumble, and thereby learn a much needed lesson.