The big attractions of non-satellite aerial platforms are cost and accessibility. Large-scale technological upgrades are also not possible. In the case of Iridium, the satellites they were using were redundant before they were launched. Using aircraft, be they unmanned balloons or specialised planes, a communications network can be placed over our heads for a fraction of the price of a satellite system. The machines can also be routinely returned to Earth for maintenance or upgrades.
Power is also a plus.
The closer a relay platform is to Earth, the less power is needed to reach it, opening up plenty more signal bandwidth. All these advantages mean there is lots of flattering government and commercial attention being paid to satellite alternatives. This region is above the service ceiling of commercial aircraft but much lower than satellites can be deployed. An operational lifetime ranging from months to years is envisaged. They can carry payloads of around 1000kg.