Culture: Moroni’s recruitment efforts are very successful. These new soldiers are probably farmers who are in hamlets, not part of the government of any large cities. The Lamanite system would offer them no great advantages. Having an elite would not affect their lives for the better and might increase their burden of providing for the government. Thus, they are those willing to support the Nephite tradition.
There are “thousands” because these are the majority of the folk. The rebels are urbanites with already-elite lineages who have the most to gain from a Lamanite takeover. The king-men could take over Zarahemla because they were most numerous in the capital; the men rallying to Moroni’s standard were most assuredly found in the fields. Of course, cities like Gideon and Jershon remained faithful; but as we have seen in the Book of Mormon, rebellion among the Nephites is city-oriented. Ammonihah and the Zoramite Antionum are two specific and recent examples.