From what I know about the airspace in the DCA Control Zone, this was almost an inevitable collision.
The Army helo's are operating wayyyyyy too close to civilian airliners at such a critical flight segment of landing.
The helo's are required to remain at or below 200' AGL and "see and avoid" traffic that is or is not pointed out to the Army crew. Two things...one is that it is pretty hard to identify an aircraft at night with all those lights being so close to the ground when you are also close to the ground. Second is that it only takes a few seconds to stop paying attention and bust that 200' altitude restriction and drift up a couple hundred feet. And this collision occured at 400' AGL. So the Army helicopter was completely at fault.
And yes, the airliner would have a Traffic Collision Avoidance System. But, Resolution Advisories (instructions on whether to climb or descend) are inhibited on approach (and below 1,000' AGL). I don't know about the Army Helo tho... In any case, both the airliner and helo were aware of each other as they had been advised by the Tower Controller of each other. In fact, the helo had been instructed to pass behind the airliner. Obviously they helo didn't do so. Why the helo pilot and crew failed to see (?) and avoid the airliner is the Million Dollar question. My gut feeling speculation is that it was simply human factors.
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