... because we won't be making any more long distance human space flight before then.
I've just returned form a 3 week class/conference on "human spaceflight and exploration" at the IRF in Sweden and the near unanimous conclusion is that the global space industry needs to sort itself out. At the moment we have at least 3 major space agencies worldwide (NASA, ESA and the RKA), all using different standards on simple things like docking hatch sizes, let alone things like objectives and policy. As there is no big enemy that needs to be shown up anymore so there is no reason to assume that any single agency will complete the next milestone, be it mars or an asteroid or even "just" a lagrange point station. ESA and NASA are both currently involved in massive restructuring programs including standardising interfaces and looking to contract out to private industry rather than build things themselves. Whilst sad from a pure science point of view it does make sense; as specialised industry develops more and more private, non governmental agents get involved, there comes the money.
The development of new systems is becoming more of an international affair and the "next generation" of internationally compatible spacecraft should (supposedly) take 10 years... then 10 to build.
As far as choosing a destination goes the elements required to get there and back and do something useful when there are the same; you need people able to hand long periods in a small place together, more efficient life support and propulsion, new "planetary" suits, and money, lots of money. The current thinking is that NASA, RKA and ESA will have to work together on any future large scale mission. Possibly with JAXA and ISRO and CNSA support, and each would supply parts or money or whatever.
One of the key speakers at the IRF said he expected that people would go to mars first and that it would happen in the next 20-25 years. He was an astronaut for ESA and most of the other experts agreed with him (including fairly high up members of ESA and the SSC). So i'm gonna go with that point of veiw.
Plus, from a purley simplistic and personal point of veiw; a man on mars would be WAY better than a man on an asteroid or floating around at L2. Why? Because its MARS! its the second most logical place for a first base on another body... its just exciting.