Pedantic point: In many cases I believe you can import them, it's just expensive and annoying to jump through the hoops. Manufacturers could even sell them, they'd just have to pay a fine/fee/whatever.

As a fan of light compact trucks, the kind they don't sell here in the U.S. anymore, my understanding is that a badly-written function in the CAFE standard calculations is one of the primary reasons I can't buy a truck that's smaller than my bedroom. The function calculates acceptable mileage based on vehicle footprint, and was nominally intended to encourage better fuel efficiency, but had the perverse effect of making trucks larger. Even then, it's legal to sell smaller trucks, but manufacturers have to pay a fine that apparently ruins the profit margin for them. And then there's the infamous chicken tax, and so on.