If you're puzzling over how something like this happened in controlled airspace it's IMO worth watching this video on YT.
In short summary:
We don't have the FAA investigation done yet, but it seems pretty clear the air controllers were doing their jobs properly, as was the aircrew on the plane. The military helicopter pilot requested visual separation, meaning that the airliner 'owned' the airspace it was about to pass through and the helo would not get in their way. The controllers asked the helo pilot multiple times if he had eyes on the plane he had committed to avoid - and he said yes each time - but I would guess that he actually had eyes on the plane behind the one he hit. Meanwhile, the pilots of the plane have restricted vision downwards - and mil helo pilots wear helmets that restrict peripheral vision.
My personal background:
I used to fly (VFR only) and as I had a Pitts (great toy for messing around in the sky) I have *zero* experience in controlled airspaces; all I know is I should stay the hell away from anything marked that way on a Jeppesen chart. Hanging around an airport for private aviation meant I soaked up the ‘lingo’ — and the ability to read a chart. So use salt with my takeaway.
My takeaway:
Granting visual separation to mil pilots is apparently routine. I suspect that's about to change. It's a tragedy that this accident happened.
Also: Captain Steve is the real deal. I've known a number of commercial pilots and he's very much in that mold.

I'm putting this here because media coverage of what happened has been so bad that I keep switching away, sort of the same way I can't listen to the whiny voice coming out of the Orange Menace.

[edits to fix spelling / typos and embed link into text to make it easier for mobile users to follow]