The Lord agrees to preserve the plates, but more than that, covenants to do so. The Lord had shown Nephi that his words would be preserved, and would play a major role in the redemption of future Israel. Enos does not appear to have known of this prophecy, based on his personal urgency to assure the continuation of the records. Therefore, he requires it of the Lord. The difference in Enos case is the covenant. Where Nephi was show the future, with Enos the Lord makes it official.