Because that is not really needed. At the current spectrum build out can be better then anything that 800mhz can offer, all they have to do is do it.
Reminds me of a the doctor off ST Voyager, "you can live a normal life, if you only choose to live a normal life."
In the next few months, wimax will start launching mini sites. This macro-cells will cost about 200 dollars, they will attach to electrical poles and street lights, and even your home, they will have the range of about 2 kilometers and they will work as a spider web, connecting all the cells together.
This is will happen over the next few years. Instead of 1 tower that can service 1,000's of people with 800mhz. There will be 1000 of macro cells servicing 100,000's of thousands.
LTE is not made for this, wimax is, and at 300 dollars per kilometer, it is a heck of alot cheaper then lte.
As, you say, lte offers nothing to sprint, except more cost.