As much as I'd like to fault Delta for this, I can't. It's not a shortage; it's an error in allocation of resources. There's hardly a "shortage" since they've got ~2K pilots too many.
Every airline is playing Pin The Tail On The Donkey in deciding how many pilots are needed...and in what seats...and in what fleets. I don't see how any airline comes up with the right answer in the current circumstances.
Some carriers have done better than others but it may very well be dumb luck. We'll never know.