Are you sure about that?
I'm with aaronvan with how confident I was in 1969 that humans would be on Mars by 2000 at the latest. Bit now, here we are almost 50 years later with still no firm plans. I wouldn't be surprised to see these type of projects perpetually put off so we can spend our money where it's really needed: endless war.
No, as I have already mentioned, prolonged exposure to a zero gravity environment takes an enormous toll on the human body. A week away - as with the Moon flights - is easily enough dealt with.
Months and years - which is what a Mars flight would entail at current levels of space flight and rocket development - a lot less so.
However, the stress on the body of extensive (meaning months and years) rotations in a weightless environment is very punishing, even if you exercise frenetically on a daily basis. Through their time on the ISS, the Russians have done some extraordinary research on such matters, - and how it effects the organs of the human body (badly, for the most part) - and, I suspect, the general conclusion is that - for now - it simply isn't worth it. Physically, we might not be able to cope with the effects of such long journeys.
By the time someone got to Mars, even allowing for the difference in gravity, they'd barely be able to crawl out of the lander.