Mormon begins to tell the story of the change in government by discussing the transfer of records. The records were part of the set of sacred items that were traditionally passed from one king to the next. Therefore, according to tradition, he prepares to transfer the plates of brass and the plates of Nephi. However, when the people of Limhi arrived, they came with their own records. Not only did they have their equivalent of the plates of Limhi, their record of Zeniff, but they also had twenty-four plates that were discovered in a destroyed land, and which they desired to be translated.
It is those plates that send Mormon onto his aside. He discusses the desire for their translation, and then uses Mosiah as the translator. All of this information is interesting and important, but none of it is directly related to the transfer of rulership. This is where Mormon inserts important information that takes him away from his intended storyline.