The fuel economy standards (CAFE) take the footprint (size) of the vehicle in count. Once the vehicle is big enough (wide in this case) it effectively falls completely outside the rules saving the manufacturer a lot of money in fees/taxes.

For imports “light trucks” get a nice extra 25% tariff (been around since 1963)

If you own a business and actually use it for work then you a bigger tax brake if it weighs over 6000 pounds.