Tom Wolfe in a rarity, a NY Times editorial worth reading, has this to say,

"As a result, the space program has been killing time for 40 years with a series of orbital projects ... Skylab, the Apollo-Soyuz joint mission, the International Space Station and the space shuttle. These programs have required a courage and engineering brilliance comparable to the manned programs that preceded them. But their purpose has been mainly to keep the lights on at the Kennedy Space Center and Houston’s Johnson Space Center"...

Forty years! For 40 years, everybody at NASA has known that the only logical next step is a manned Mars mission, and every overture has been entertained only briefly by presidents and the Congress. They have so many more luscious and appealing projects that could make better use of the close to $10 billion annually the Mars program would require. There is another overture even at this moment, and it does not stand a chance in the teeth of Depression II.

The guy writes it better than I can.