Helicopters are orders of magnitude more heavier, expensive and difficult to operate. And they are very vulnerable to an opponent damaging the props.

Drones in contrast have much less payload and range, but they can be made extremely cheaply (a DJI Agras T100 spray drone with about 75kg payload is ~13.000 €!), more difficult to hit due to lower size, and anything above 5 arms is pretty resilient to prop damage.