Moon/Mars
The most plausible explanation I've heard comes from my friend Gordon Pusch who pointed out that Secretary of Defense Rumsfeld seems to have bought into what has been called the "Vision for 2020" -- a space age "Project for the New American Century" -- that calls for U.S. military superiority (and exclusivity) in space. This goal requires (and does not yet have) a"heavy lift" capability into space: launchers that could put massive payloads into orbit. (The Shuttle won't lift enough payload, and can't launch frequently enough.) A few billion won't get very far along the road to Mars, but it will pay for launcher development. And, as it happens, heavy launchers would be the first thing needed by the Moon/Mars program. Moon/Mars is a lovely" civilian" cover to develop these heavy lifters, which otherwise can't be justified -- weather and communications satellites need only small launchers.
With heavy lifters, the U.S. could then deny the use of space to other nations. But to militarize and to enforce a monopoly, the development and operation would have to be under U.S. government control: thus, NASA.