A big part of the problem is that the used market is controlled by new car buyers (from the past) - if nobody is buying new compact/cheap cars (for whatever reason) then they're not available for used buyers.
And you can get what has happened multiple times - when gas prices rise substantially, low-mpg vehicles get dumped on the used market, so you have poor people and kids driving massive boats (the hotrodding scene from the 70s is a direct result of this).
