Unfortunately, there is no singular incentive to entice the majority of U.S. citizens to be willing to foot the bill for a manned Mars mission.
Bear in mind that I was a former astronomy student and though I no longer pursue that as a profession, it is a field that I am deeply interested in...particularly the manned exploration of space.
We went to the moon to gain the ultimate 'high ground' over the U.S.S.R. Yes, there was a lot we learned there, and sending men rather than probes proved fruitful in many respects, but as soon as Armstrong took that first step, we beat the Soviets. Everything after that was gravy. We got their first, the U.S. public got bored with it, the program got axed with Apollo 17.
Sending men to the moon again would be even more costly than it was in the 60's and 70's. There are no more Saturn V's left to ferry men up to the moon. The few that remained after the cancellation of Apollo are museum relics. Everything would have to be started from scratch. The STS system is unsuitable for anything but low earth orbit. Besides, there is no way a shuttle is going to land on the moon, much less take off again.
One could argue then that we simply move forward and plan for going directly to Mars. Though many have formulated some viable plans, like Dr. Robert Zubrin with his 'Mars Direct' proposal, it would still be incredibly costly and require the creation of some new technology. Again, it would be difficult to convince the American populace to pick up that check.
The obstacles of a manned Mars mission are considerable. At best, it would take around 6 months...one way. En route, the crew would be at risk of radiation exposure whenever the sun decided to puke on them. We already know that living in a microgravity environment for a long period of time causes the body to weaken considerably. The crew risks further radiation exposure while on Mars, since the Martian atmosphere and EM field are not sufficient enough to protect them from solar outbursts.
I think I'll stop now, because entire books have been written on the difficulties of a manned Mars mission, and I could not do them justice here. I would love to see a manned Mars mission...hell, I'd love to be on one. Unfortunately, I don't think the current social, political, and economical climate are conducive to such a venture.
Cthulhu