Hoover, over on YouTube, on a series called Pilot Debrief, speaks of a model where "all the holes line up in the Swiss cheese"--that is, a series of small events line up to produce a catastrophe.
I think this may well be one of those sort of events.
The plane crew was likely busy as all get out configuring the plane for final approach, concentrating on that task, altitude, airspeed, and alignment with the runway. At their altitude and configuration and airspeed, any abrupt maneuver to avoid collision (had they seen it coming) would likely have caused an unrecoverable stall, with the plane going down and likely no survivors.
The Helicopter crew was likely similarly focused on their tasks, had noted the other plane in that airspace, but was not aware there were two. (the ATC was not completely clear on that point, and may have missed an opportunity to have the helo reverse course in time, but that is merely speculation)
I would not want to be in their shoes, whatever the case. Rather than rush to judgement at this point, I will pray for all involved.
I will wait until there is more information and see what the case is.