Recall that research spending on SUVs is quite modest. Hence one ends up with a tough looking, commercially attractive shape that cost little to no windtunnel development. The larger size of SUVs also lead to an increased frontal area. It is easy to see how an increased frontal area leads to more drag. The high ground clearance needed for off roading also means that there is a great potential of the air travelling under the vehicle to become turbulent.
Turbulent air leads to increased drag.
Since most SUVs spend time on paved freeways at speed, all these combined aerodynamic debits add up to significantly increased energy required to push air out of the way. A particular variant of the Range Rover offers air springs whereby the ride height of the vehicle could be adjusted depending on its current mission. Using slightly more advanced technology like this would mean that a SUV could potentially excel both on and off road, but the market has shown very little appetite for excellence of this sort. A cab and a bed is then installed on top of this frame. A biological analogy to this design is a vertebrate such as a human being, where the skeleton is concealed inside the exterior surfaces.