This was discussed by the retired Army helicopter pilot on the Opposing Bases podcast. To paraphrase, his response was that civilians aren't so keen on helicopters coming to a hover at < 200 over their house, business, boat, etc. I'm sure there's some helicopter pilots here that can confirm, but he also said in the military they're trained to avoid coming to a hover over civilian areas for those reasons. Civilian complaints seem to be true, based on our old neighborhood Facebook cluster**** group where many were keen on complaining about any "scary" helicopter.
They did say that if the controller had detected earlier that something was amiss they could've had them slow down considerably or do left 360 away from DCA.
One thing they said that really stood out to me is, because of these helicopter routes and the regularity of how they were used, the controllers probably didn't think anything was wrong until too late. Since they're accustomed to running these routes close to approaching airliner traffic and seeing that proximity on their scope, it looked in the realm of "normal" to them. Just one more hole in the Swiss cheese.