Although Mormon does not specifically say that he continues to use the record of Zeniff as his source, that is clearly the case. As with other official records, the record of Zeniff’s people would have been recorded on a record named for the beginning of the dynasty, and subsequent rulers’ records would have been included in the same book. Therefore, although Mormon ceases quoting from the record of Zeniff, he continues to use that source for his information.
Why does Mormon stop quoting? A plausible answer is that an official record would put the next king, Noah, in a favorable light, and Mormon doesn’t see him favorably at all. Mormon narrates so that he can paint his own picture of Noah.
That picture begins in the first verse where Mormon notes that Noah “did not walk in the ways of his father.” Mormon then creates a catalog of the things that Noah has done contrary to the commandments of God. The first is that he had many wives and concubines. This is contrary to the law given to the Nephites, as recorded in Jacob 2:26–27. As noted in the comments on Jacob 3:12–14, one of the mechanisms for gaining wealth was based on family production that was enhanced by larger families, including more wives.
The idea for how this might work was likely the result of witnessing other peoples in the land. Thus, the people of Zeniff had moved into a very Lamanite land, and would have had the examples of both wealth and how to obtain it all around them. The desire for multiple wives and concubines is more likely to have been related to wealth and prestige than to any other motivation.