For one, just as I speculated, the Black Hawk was in a nose-low attitude, which you can see based on where it's rotor beacon is in relation to the lights on the first of the helicopter. That means their upward visibility was very limited. If they were wearing night-vision goggles, which were reported by the Department of Defense as being on board, it would have been even more so. That's the first major issue.

Secondly, while some keep referencing how bright the landing lights were, those would not be very visible to an aircraft approaching from the side, as the Black Hawk was. That's especially true on a clear, dry night where there's not much moisture in the air for the lights to project on and reflect back. There is also an enormous amount of light pollution in the background for beacons and strobes to get lost in.

Thirdly, and perhaps most telling, is that the regional jet was descending on the final approach path after making its base-to-final turn while the Black Hawk seemed to be in a slight climb (according to the track data). That means the two aircraft were not at the same altitude until the moment of impact. In other words, the regional jet essentially descended into the flight path of the helicopter, with the latter's crew having little to no upward visibility.

As a point of clarity, the regional jet was where it was supposed to be. The Black Hawk's flight path was the issue, and it never should have been where it was. Further, given that the helicopter appeared to have called the wrong traffic in sight when warned by air traffic control, it's also very possible they were distracted and not even looking in the direction when the collision occurred.

The last question (one President Donald Trump himself has raised) revolves around why the Black Hawk was at the wrong altitude, and there are several possible explanations. You can gain a hundred or so feet of altitude in an aircraft in the blink of an eye. That could happen due to a lack of proficiency. It could also happen due to being distracted.