It's about CYA and the bottom line for U-Haul. It's easier to blame the lawyers than for them, U-Haul, to actually look at the change from body on frame to unibody and change the policy for the newer Explorer. That plus U-Haul corporate doesn't have to be concerned about whether one of rentees/dealers knows the difference in design and/or be concerned about the year of the Explorer. It's expedient just to keep the ban on all Explorers.
I'm not buying all of U-Haul's drivel here. They could change the policy with the new Explorers if they wanted, they're using yesterday's experience, to blame keeping the policy the same for the new Explorers.
fwiw, when my son purchased his 05 Explorer one of the first things he told me was he couldn't rent a UHaul to move his stuff with it.