Interviewee on the BBC a few minutes ago suggested, as above, the potential for the helicopter crew to have mistakenly identified the aircraft departing (on further back on approach) rather than the CRJ they hit.
On the face of it, that seems stupid but is, I guess, entirely plausible as others have noted above (angles, spotting the specific dot of a moving light!).
But that ignores all of the other systems which could prevent such an incident. Did ATC switch attention to something else, and miss the opportunity to intervene when it became apparent that both aircraft were getting close. Etc.
Seems baffling that this could happen in such a tight controlled environment...