I am sorry you had that experience. I have taken that flight many times since 9/11 and never had that problem before.
The problem as someone stated before, is since this is a small airport, some people think that you can show up 30 minutes before, and you can make your flight. Which used to be true in some cases.
The only reason why that supervisor would have done that, is that we only have one non-stop flight to Chicago a day, while there are many going to DFW
I fly mostly DL, AA and SW out of ELP, and never had that experience before.
Sorry you had a bad experience in the wonderful "Sun City"