Since the helicopter hit the slack line, it's very likely that the slackline was against the sky and was a similar brightness as the sky behind it.
Had the helicopter been at a greater altitude the slack line would have been very visible, as it is in the photo in the NTSB report. I think a similar problem happens with the LEDs and the wind socks - the contrast is insufficient to be readily noticed against the sky by a pilot who is likely concentrating on the distance below the aircraft and to surrounding mountains.
I have no doubt that the concern was empirically correct on every other trip, but it does suggest that the FAA has not provided sufficient guidance or research over what marking is sufficient.
I'll just repeat - the FAA needs to implement a radio based system of alerting that can be used by ground operators to mark temporary and permanent obstructions and to mandate the use and make it required equipment to receive and give audio and visual feedback to the pilots when proximity is computed. Instead of just a NOTAM the FAA could require transmitters that would convey suitable data, making for a rental market for such temporary things as tower cranes or logging operations.
The guy in New Zealand has recently upgraded his system that does this sort of detection to not only include proximity and altitude, but also give voice outputs so that a drone operator with First Person Video glasses doesn't have to look at a receiver screen to be informed of a potential conflict with manned aircraft. He's one guy, a hobbyist, with a small budget. Why the FAA won't or hasn't moved forward on this boggles the mind.
For Beta software, it does the job.