The Need for Government Support
While the commercialization of space continues at a rapid clip, as with all business it is driven by profit motives and risk/reward ratios. As such, very little is done in space by private ventures that has not been tried and tested before by government agencies. The costs are too high and the risks are too great for investors and businesses. In situations such as this, when there are clear benefits to society but the risks and costs are too great, the government steps in to support the effort and spreads those risks, costs, and benefits across the whole of society. Obvious examples of this are health, medical, energy, and basic science research.
The human exploration and development of Mars is just such an instance. While the benefits are many and great (see Why Mars? in Adobe PDF format), they are primarily scientific and societal. The economic benefits derive mainly from potential space-based industries that do not yet exist. While the costs of human Mars missions are too great for private initiatives, it would be a VERY small fraction of government spending with incredible benefits.