Anthropological: We are now faced with a fascinating conundrum. We have a small number of people straggling out of the wilderness, and suddenly find the leader of that smaller group as the king over the established larger population in the city of Zarahemla. Why is Mosiah made king?
As with most of these reconstructions, we are in the realm of speculation, but the combination of factors that come together in placing Zarahemla in a Mesoamerican milieu in a particular time and place can provide some interesting hypotheses. As a preliminary, we must return to the plausible location of Zarahemla at the archaeological site of Santa Rosa (Sorenson's correlation).
First we must understand that Santa Rosa has a history of occupation in one form or another dating back to 1000 BCE (Delgado, Agustin. Archaeological Research at Santa Rosa, Chiapas and in the Region of Tehuantepec. BYU, New World Archaeological Foundation. 1965, p. 79). Thus when the Zarahemlaites enter the picture, they are moving into an area already having some organization and structure. Santa Rosa does undergo a marked development in the late Preclassic period, however, which time period covers both the Zarahemlaites and the Nephites. It is during this period that the site is "characterized by its advanced architecture, known to be of imposing dimensions in at least a few instances. Typical are stone walls and sloping batters (talud) covering earthen fills, and the use of floors of tamped, sometimes burned, clay. (Delgado 1965, p. 79). The florescence of the site will occur later, however, in the Protoclassic, (Delgado 1965, p. 79) which in terms of the Book of Mormon makes sense as the new Nephite nation begins to regain power and importance.
The next most fascinating aspect of Santa Rosa as a candidate for Zarahemla was noticed by John L. Sorenson. One of the temple mounds has a remarkable feature. Delgado simply notes: "The plaster floor continued in both trench extensions. In contact with it, both above and below, was a thin layer of gravel. That below was of different natures to either side of the medial line of the temple. To the north it was composed of larger fragments of broken stone, while to the south it was natural gravel. The difference was probably due to different sources of material." (Delgado 1965, p. 29).
Another archaeologist working at Santa Rosa expanded the analysis of this unusual gravel sub-floor: "To the north the gravel was broken and to the south it was rounded. I supervised that excavation and, upon noting the difference, carefully searched the gravel, finding no mixture whatever. Not only does the difference suggest two sources of materials but it may be taken to imply two separate groups, each working on its section. Further, the medial line runs roughly east-west." (Brockington, Donald L. The Ceramic History of Santa Rosa, Chiapas, Mexico. BYU. New World Archaeological Foundation. 1967, p. 60-61.)
Of course one cannot be certain that we are dealing with two differing political groups, but virtually all sites would represent more than one kin line, and kin groupings do not seem to be an adequate explanation for the separation of types. The east-west median is certainly explainable as the transit of the sun, but in the possible context of the meeting of Zarahemlaite and Nephite, the segregation into a north section and a south section, given the descriptive differences between the homelands of the two parties, we have an even more fascinating possibility.
Another interesting facet of Santa Rosa comes in the ceramic evidence. Santa Rosa has its own ceramic styles that change through time, allowing for rough dating to types that were popular during certain times. In addition, the ceramics do show some correlations to ceramic styles from other sites. In the time period in which the Nephites would have arrived at Santa Rosa, certain pottery show ties to Kaminaljuyu - ties sufficient to be termed "perhaps the closest linkage of our material to other regions." (Sanders, William T. Ceramic Stratigraphy at Santa Cruz, Chiapas, Mexico. BYU. New World Archaeological Foundation. 1961, p. 53).
Remembering that the plausible location for the City of Nephi was Kaminaljuyu, the close connection between pottery styles suggest some type of correspondence between the two sites. Given the separate culture and language between the two, the correspondence would be surprising except that in the Book of Mormon context it is completely understandable.
Thus there is reasonable evidence to suggest a tie between Santa Rosa and Zarahemla. Assuming that tie, we can use that information to assist us in understanding the way in which the Nephites and Zarahemlaites merged, and the reason for the selection of Mosiah as the king. Kaminaljuyu at this time period is both larger and more wealthy than Santa Rosa. If the impetus to the improvements in buildings is related to the arrival of the Nephites, then Mosiah's people would have arrived at a town that while populous, was not nearly as well built nor elegant as Kaminaljuyu. Delgado notes that while Santa Rosa might have been grander than its nearer competitors, it was "rather poor when compared with Chiapa de Corzo." (Delgado 1965, p. 79). Chiapa de Corzo might be closer to the development of Kaminaljuyu.
Using these real world sites as a background to the Book of Mormon, we have the City of Nephi becoming more and more wealthy, and prosperous in the ancient world. At some point, that prosperity causes either the internal or external pressure that forces out the believers who follow Mosiah. Since that tradition of belief traces clearly to the teachings Jacob, we are justified in suspecting that Jacob's antipathy to the accumulation of worldly wealth would have been continued by Mosiah's believers. Thus these are not necessarily the wealthy who leave, but certainly Mosiah and others would have been part of the upper eschalons of the city of Nephi. When they arrive at Santa Rosa they have come to a lesser location, a place of fewer riches, fewer fine buildings. With their beliefs this would not necessarily be a major problem (and particularly compared to the necessity of finding a place to live). Therefore we have the Nephites coming from opulence to relative poverty among the Zarahemlaites.
In this context, we note that Zarahemla is not called a king, yet is clearly the leader of the town. This is a distinction in the type of rulership, and it may be that the Zarahemlaites had not yet grown sufficiently independent and powerful to adopt a king as a mode of government. Mosiah, however, came from a tradition of kingship. As has already been noted, Mosiah carried the sacred symbols of power, and his possession of the plates indicates some relationship to the ruling line at the City of Nephi, although (again as noted before) he probably was not the king in the City of Nephi. Mosiah's arrival from a more powerful location, his immediate connection to a regnal line, and his demonstrable ties to ancestral right of rulership through the evidence of the plates are all factors that would make his selection as king logical, even in the face of a larger population of people with a foreign language and customs.