Blame the obama CAFE regulations that accounted for wheelbase and car volume, giving manufacturers lower fuel economy standards for larger cars. Then the CAFE standards that hold trucks/SUVs to a lower standard.

The economically efficient way to get the fuel economy result would have been to increase gasoline taxes, but that's a non starter politically. Higher gas prices would allow people to choose to keep a cheap gas guzzling truck/car, buy a new more efficient and expensive car, or buy a new slightly more efficient slightly more expensive car. It would have been simpler though and given consumers more choice.