These verses suggest that the reason that the brother of Jared’s account should not be written until after Christ had shown himself referred to Christ’s appearance in Bountiful, and not to his mortal mission in the Old World. The separation of the hemispheres would have kept the New World writings from those in the Old World whether they were commanded to be written, or only revealed at a certain time.
Verse 3 flows from that meaning. The experience of the brother of Jared seeing the pre-mortal Messiah was written after Christ appeared in Bountiful, and, eventually, that people to which he had revealed himself “all dwindled in unbelief; and there is none save it be the Lamanites.”
Keeping the record from the Nephites made their experience with the resurrected Christ a marvelous, and somewhat unexpected event. Knowing the experience of the brother of Jared would not add to that majesty. However, now that the record was being prepared for a future readership, it could serve as another testament to the God who condescended to earth to become the Messiah.