It sounds like that yesterday being the one of the busiest days of travel and weather messing up travel plans, sounds like the flight was way oversold in Y. My guess is that they were simply instructed to not move anyone to that flight as I'm sure the standby list was pretty deep and you were just one more to the pile.
Having said that, I agree with you to a point. I find that AAngels can process upgrades, make seat assignment changes, and take care of most things in the club, whereas DL usually can only call the gate and make a request, especially if it's within an hour of scheduled departure. But I have also found that the DL gate agents, with the exception of DTW, will simply take care of it and I get a new boarding pass or whatever request was queried.