Are there any geo areas where ATT+TMO/Sprint (combined due to merger) isn't at parity with VZ?
I think what happens is that tower leases in some protected areas can be hard to negotiate, so the cost/benefit ratio gets out of whack for new towers (amongst other issues with power, construction, and RF propagation in remote, mountain areas). Maybe someone with more knowledge can chime in.
In some locations, there just isn't any land to site the tower due to regs (national parks comes to mind, nothing except my Iridium hotspot works in parts of the Smokey Mountains), so there are no providers or a single provider who was able to swing a special case on an existing tower (water tower or similar).