Towers are typically owned by a REIT (real estate investment trust) or other entity that develops the tower and telco/equipment shelter and then leases out space on the tower to mobile providers (cost is higher to the mobile provider the higher you want to be on the tower).

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).