I think it is because they are so far away from the terminal that the airport authority has no control. It's adjacent to private parking lots, sex shops and what you there off Century Blvd. Just a theory. I think the second tier companies like Enterprise offer "off airport" rates at some airports because their location is not within the confines where they are regulated. At DEN I imagine that this would be impossible unless it's like 15-20 miles away.

Some airports have these horrible communal locations that are miles away but again it is controlled by the airport authority. ABQ comes to mind.