It’s likely both the helicopter and the jet received a TCAS warning. In dense airspaces, those alerts tend to trigger frequently, so there’s a strong chance they may have dismissed it. The CRJ crew might have been aware of the Blackhawk’s presence, but if the other crew had visual contact with the approaching traffic (the CRJ), they might not have felt the need to take further action.

TCAS RAs are inhibited at low altitude.

I've seen this comment widely. But...why doesn't TCAS show the pilots at least as much information as I would have seen on my phone if I'd been browsing flightradar at the time? It's one thing to design a system with "don't saturate the operator with audible alerts" but another to say that it's appropriate to withhold very valuable information from the operator ("there are loads of planes in the general vicinity, but yeah look at that one coming at you from the left").

the TCAS data would have been displayed. but not on instruments or in any place that you typically pay attention to the most during final visual approach (ie., mostly outside).

Too low, most likely.

as others have said, likely too low. tcas is pretty awesome (and yes, most military flights will use it), but it is (deliberately) muted/degraded at low altitudes (i think around 1500feet).