If they offered it from an East or West coast city (IAD or SFO/LAX) I suspect the flight cost would be higher than $129 and $229. I am surprised they did not restrict this to one segment nonstops.
Denver works (like IAH might) because of the non-stops. This is really not mileage run fare. You would have to do lots of trips say from DEN-ORD (or IAD) to MIA and back.