A SUV is going to weigh more than a similarly sized pickup truck for starters and the extra passenger space of the SUV reduces the trailer portion of GCWR by 150 lb per passenger space in the SUV.
There is also a difference in many cases with the rear suspension with the SUV having a design for maximizing passenger comfort whereas the truck is designed to carry as much of a load as possible.
In general the engine cooling and trucks' brakes have improved a good deal over the years. Some of this is adopting disc brakes at the rear wheels and in others it has come from larger rotors at all corners.
Frames have also been strengthened and with the current heavy duty trucks a WDH is not required as with the 1/2 ton trucks and SUV's.
