DL and a number of other carriers use a third-party vendor. The software, which is proprietary, looks at a wide variety of factors to determine who is a risk. Departure airport being in a high fraud area is one. But, there are many others.

Hate to say it, but some number of factors have combined in your case.

You are by no means assured that DL will cancel the ticket if you no show. So, a real risk to count on that as a means of making non-refundable into refundable.