There are 4 random plains to balance out the 4 city maps - city maps favour infantry, random plains favour cav, it's only fair for them to be in equal numbers. Also I'd count ruins and field by the river as open plain maps, so there would be 12 plain maps, but compared to random plains they allow everyone to have fun, not just cav.
I think the native village maps are the best compromise since they are generally good for all classes. They offer some cover for infantry so cav doesn't dominate while still being open enough to allow cav to be perfectly viable. Melee have the tight alleys and closed areas, ranged have roofs but firing zones aren't too good and with the reworked maps, both teams reaching the village at the same time rather then 1 camping while the other charges keeps it generally balanced.
