Here's an excellent video from Juan Browne around the challenges that wires present to aircraft operations [1]. Some of these are human factors for manned aircraft, like seeing a wire but then forgetting it's there, but one of his points is that it's simply safest to avoid flying below 1000ft AGL. That's not an option for drones today, and they presumably don't (yet) have the ability of humans to make inferences about the likelihood of cables near cranes and transmission line towers, making them particularly vulnerable.