A lot of it as mentioned has to do with marketing and sales. If they introduce a new model with evrything on it and it never changes, then they sell X number and then sales level off or start to decline. When that happens they have go to items they introduce to get people to buy a newer model. Many of us on here will regear, supercharge, tune, etc. to make our trucks what we want However, the vast majority of buyers would rather just trade on a newer model to get the "newest goodies".
With that said, the new 8 speed with the same 3.73 and 4.10 will be way better than the 66RFE. Therefore, most people will go buy that new truck instead of adding gears. Its marketing. They care about sales more than anything. And as I said, we here on the forum are a small sample of truck buyers in the world.