Our modern Chapter 6 in the book of Mosiah was a chapter in the 1830 edition. It is rather unusual in that it is so short and covers so little information. When the next chapter begins, it will begin telling the deeds of Mosiah. In this chapter, Mosiah is mentioned, but nothing of what he did, other than that he “did observe [the Lord’s] judgments and his statutes, and did keep his commandments.” That is very generic information.
There is no simple marker to tell us why Mormon created a chapter break at this point, but it appears that this was the intended ending for the events Mormon discussed in Chapter 5, but which were separated from that chapter due to the testificatory Amen.
The key to the conclusion is the last sentence. “And there was no contention among all his people for the space of three years.” The entire purpose of Benjamin’s speech and abdication was to counter the contention that had gone on before. This sentence functions as a testimony that it worked. In the timeframe of peace in the Book of Mormon, three years was a respectable amount of time. Years of peace will continue to be marked, and are often shorter than these three years.