I doubt that area is a main factor. If that would be it then Russia would have higher prices and less subscribers, yet SIM card penetration approaches 170% (see http://www.budde.com.au/Research/Russia-Mobile-Market-Insigh...).
Anyway, it's a moot point. Rural areas are cheap to cover. Stick a macrocell up, microwave some backhaul in from another tower (until you hit the nearest fibre line) and jobs done. Cities are much much more expensive - costs more to put the masts up, need many many more masts and complex frequency allocations, buildings blocking the line of sight, etc. I'm sure that rural capex/opex is a very small percentage of Verizon/AT&Ts budget compared to all the thousands of new 4G sites they're putting up in urban areas.