I'd have assumed it was because of the ability to see code share inventory, but she had said first that she would have to call and then basically refused to call.
Though she did call Delta.

I would have thought that protecting me on the flight to IAD (which is a co-terminal with DCA) would have been the easiest thing to do, which is why the whole thing struck me as weird.

I should note this is the first time in months that a delay was announced with time to do something about it. My more typical experience is what happened Friday night when I got the United call / email at 9:15 p.m. telling me that my 8:59 flight was delayed until 9:20. (It really left at about 9:40, by the way.)