> Why is the latter worse than the former ?

I'm not sure about worse, but think one of the differences would be the size of the 'kill zone' and the cost/availability. 10 quadcopters "cover[ed] between 3 and 5 kilometres ". That would take a lot of bombs and a multiple aircraft sorties with to kill everything there. e.g. During the Vietnam war, a group of 6 B-52 bombers modified for carpet bombings could bomb an area around 1Km x 3Km. Only the US and Russia have heavy bombers that can do that. It could be done with smaller fighter aircraft, but that's more sorties.

That's vs. 10 quadcopter drones.