That's a good question.....
And it's not just in the past couple years either. I bought my 07 dually in August 2006. I grumbled about the same thing. And my 03 before it. Seems to me it was either 54 or 57 inches to the top of the box sides. Why?
Never understood the need to have it so high off the ground that you could fly a small airplane through the enormous gap under the truck.
I actually looked in to having the whole truck lowered but the logistics of it made me decide not to. The design is such that lowering it would cause all kinds of issues.
Getting older now and with an ugly back injury and a really messed up shoulder, I now keep a 3 foot stepladder in the back so I can get in and out of the back without blowing a vein or something.
Yeah, good question you posed here....