CAFE rules are the real reason - fleet wide emissions targets means lots of more efficient cars allows ‘indulgences’ in the form of limited numbers of very high margin larger vehicles. See the ford raptor for another example.
Once a vehicle is over 8k lbs, its considered a commercial vehicle. which is why all 3 major companies '3/4 ton' pickups are just a hair over 8000lbs..
...with a whole host of other tax breaks that kick in, including depreciation, "investment", "infrastructure", or whatever the economic-stimulus du jour is.
