It's the technical part:
" Two more differences are more than nuissances, the impracticality of evolving orbital launchers from low speed cargo rockets, and the impracticality of incremental testing for many configurations. Airplane developers were able to learn by building economically useful low performance airplanes before moving on to faster and higher performance designs. But there is very little market for a reusable cargo rocket that doesn't make it into orbit. The process of testing a new vehicle is analogous. An airplane can do taxi tests, then low speed flights, and gradually "expand the envelope" to find out if there are problems before committing to performing at the limits it was designed for. The only rocket I know of that was able to do this was the DC-X. A VTOHL rocket that lifts off the pad is committed to making the transition to horizontal flight in order to land safely, and has no way to abort testing of the transition maneuver."

And also, an interesting part:
"At one time it was often claimed that the Shuttle would reduce costs by flying frequently so as to take advantage of economies of scale. Nowadays it is generally thought the Shuttle can't fly more than about a dozen times per year without either compromising safety or greatly increasing staffing levels and perhaps capital investment. However, it would appear that economies of scale are part of the reason why Russian launch vehicles are relatively cheap--they've built a lot of them and lifted a lot of relatively heavy satellites with relatively short operational lifetimes. The Russian vehicles generally use larger numbers of engines than comparable American vehicles, but fewer different kinds. (Note the similarity to the Microcosm Scorpius design.) Maybe the relatively modular Russian expendable boosters really are dramatically cheaper than Western launchers in terms of man-years per ton of payload, and not just because of exchange rates."

If there are potical reasons (I haven't seen many in tha article, it's not NASA that's to be blamed, but the politicans.

Also, comparing with what's done in other countries and their lack of success in dramatically reducing the costs, is an indication too.

Raoul