Because they were a large fraction of cars manufactured by US companies, so not excluding them would have put the entire US auto industry out of business.
The original idea was that they needed big engines and bad aerodynamics to be able to perform their functions of hauling bulky loads and towing heavy trailers. Few people who didn't actually have those needs would want to drive one because they were unwieldy to drive and uncomfortable to be in relative to cars, so such an exemption surely wouldn't be widely exploited.
