Why would you use a heavier-than-air craft to essentially hover? Wouldn't an aerostat accomplish the same goal at a much lower cost, and lower risk of bodily harm should it fall from the sky?
I don't know why they chose it. Here's my take:
An aerostat requires tethers, which are points of failure, and has enormous wind drag. Lose the tether(s) and you lose control. Then you have a large, failing, floating device at the mercy of the winds, dragging first broken tethers, then its own large structure, on an uncontrolled path along the ground, wreaking unknown havoc.
A powered heavier-than-air (but still ultralight) has little drag and can also be made to change locations easily. With good design, if it begins to accumulate failures that jeopardize its continued operational ability, it can be made to fly to a repair site and land - after its backup has arrived to take its place.
If you have catastrophic events - like huricaines, tornadoes, or forest firestoms - it can easily be moved away (to land for shelter or fly around or above the storm) and brought back when the environment is calmer. You don't even have to take it out of service. Just fly it above the tropopause. The stratosphere is probably a good place for it to operate anyhow: Negligible weather, no cloud shadows for solar-powered planes, and gives you a lot of coverage per drone. (Balloons can get there, too, easily. But 50,000 feet or so is a LOT of tether.)