One reason we are not given knowledge of the mysteries of godliness is that we don't ask for it. Many of the sections of the Doctrine and Covenants were given in response to specific questions that Joseph Smith asked of the Lord. Granted, it is important to know what question to ask the Lord, but if the ancient Jews had inquired as the whereabouts of scattered Israel, we have no doubt but that he would have told them. But they failed to exhibit faith enough to ask. At the last day, some of us might murmur, "why didn't the Lord tell us that earlier?" The reason, of course, will be because we never asked.
This verse also shows that the Lord, in his mercy, makes provision for us—even in our faithlessness. In the case of the unbelieving Jews, he brought forth the Book of Mormon so that later generations might, if they would believe, know about the Nephites, know about the other tribes of Israel, and be brought to a knowledge of me, their Redeemer.