IMO the GVW is set where it is to keep it under the next tier which would cost people more money in licensing fees, and require hardware upgrades. When i purchased my 2017 F250, the only real difference between it and the 350 were the springs, and truck rating. My gas XD is 8800, same as my F250 was. The XD tows better IMO, but squatted worse before airbags (it's the Pro4x) Our XDs are b2 (3/4 ton) due to weight alone, even if they aren't designed or designated as such. Although most XD gas models would technically qualify for 1 ton status according to payload capacity, in some states this would cost a lot more for fees, and insurance. Not to mention we also would require 8 lug axles, etc.etc. Not saying the XDs can compete with a modern 1 ton truck, just that Nissan is playing in a weird heavy half space and most likely under rates their trucks for some of these reasons. Those of us who know these trucks, tend to like this setup... myself included.
Some interesting differences from the regular Titan I learned recently....
Different:
Steering system
Front suspension
Axles front
Axle rear
Brakes
Frame
Wheel Stud Size (wheels not interchangeable)
Cooling system (gas motor)
Intake system (gas motor)
transmission tuning
There may be more, so feel free to elaborate.