A couple things...is it non-stop? The days of Cincinnati being a real hub are long gone but Delta still offers more non-stops than anyone else. The one thing that people are still willing to pay a premium for is a non-stop.
Delta can also just be expensive. I'm going on the Disney Wonder out of New Orleans in February. I booked on Delta because I had a companion pass to use. I paid $400 for my ticket, used my companion pass, and used 42,000 miles each for the kids all from Atlanta. Pretty expensive. My parents, also from Atlanta, booked on Southwest for $250.
Another thing to realize too though is that Delta along with the other legacies these days aren't as interested anymore in price matching to hubs. The reason why a short flight from Nashville to Atlanta is expensive is because in reality, they don't want you on that flight. If they give up that Nashville to Atlanta seat for $50 then they have given up the opportunity to use that seat for someone who wants to fly a higher profit itinerary. You can't sell someone a $1000 ticket from Nashville to London if you already gave up the seat they need to get to Atlanta for $50. So if you're going to occupy that seat then you're going to pay for it.
One last thing I'll say about Delta...they don't cancel. They have a very low cancellation rate. Your flight might be delayed...really, really delayed but they still tend to run the flight. This is really great, especially on a non-stop. If you get cancelled then you're garbage. You've already been disrupted and now you and everyone on your flight can fight for the remaining seats on whatever flights are left. But if they still run it then you still have a confirmed seat.