It all comes down to weight. Your truck is probably 1,000# heavier than a 2 wheel drive long box reg cab. All the goodies you have like 4x4, 2 extra doors, ect weigh more.
The HD tow package they have available in the basic truck gives you heavier axles with 7 lugs, heavier suspension, ect. Its really about the same truck as a 3/4 ton gasser of 10 years ago. GM also makes about the same thing.
Chris