OK, DL has unused aircraft that can make the TATL trip, but yields tend to be low on mouse routes. Willingness to pay just doesn't compare to many other (TATL) that otherwise might be equivalent, perhaps because families are traveling and if one or two adults greatly benefit from extra space, for example, they might not want to pay for that space for every kid too. There will be lots of package deals and discount tickets purchased through OLTAs, while if prices aren't perceived to be cheap enough, folks will think about a different location for their vacation. Just because there are customers doesn't mean that DL should serve the route.