As noted at the end of the previous chapter, the final verse of our Chapter 28 was originally at the beginning of this final chapter of Mosiah. It began with “after king Mosiah had done these things” (Mosiah 28:20). The current first verse begins with “Now when Mosiah had done this.” This strange repetition was required to recover from the inserted aside about the translation of the plates of Ether. That information was important, but is not the reason for this particular chapter. Thus, Mormon ends the transmission of the various plates and begins in earnest to tell the important story of the shift from monarchy to judges.
When Mosiah asks the people who should be their king, he is probably following a typical process. While we do not often see the full process, it appears that the voice of the people was a part of the change from one king to another, even when the presumptive heir was clear. Thus, in Mosiah 7:9 we see that Noah was made king after Zeniff by the voice of the people. As the son, it was clear that Noah was to become king. Nevertheless, the voice of the people came into play.
We see the same action in these verses. King Mosiah asks for the voice of the people, and as expected, Aaron is declared the rightful heir. The problem was that Aaron was gone. Not only that, but any other of Mosiah’s sons, who might have fulfilled the role of heir, were also gone. Now the governmental succession was officially in crisis.