I'll take a shot, but this this is just a guess.
The 8HP70 has a very low first gear, around 4.7:1. They probably figure with this kind of torque, they can get away with the 3.21, and keep the mpg higher.
They still offer the 3.92 for the guys that want/need the higher towing capacity.
Again - based on nothing solid. If someone has better info, great!