If Delta is consistently more expensive, then there's a reason, and it's not a fluke.
People are paying more to fly DL on that route, so why should DL match WN's prices?

It seems like you prefer DL but maybe the price difference is a little too much for you. For others, I guess it's not.

I don't have any facts, but I will guess that overall, on average DL is more expensive than WN on shared routes. WN is a slightly different business model, different clientele - I would not be surprised if they priced cheaper.
The question for DL is how much more to price their tickets to attract pax and still make money.