It's the same reason major sport venues don't drop their prices to $1 right before the game to make some extra $. In doing so, you condition people not to pay the retail price and cannibalize your own business.
Like with all things, some essentially do. The Nats sell $5 tickets in the hours before the game. Lots of people just buy them instead of buying tickets beforehand. And everyone walks out happy.
Similarly, many airlines have no problem selling last minute upgrades. Delta just isn't one of them for this particular route.