Because the combination of his race, background, age and charisma make him the candidate best able to both do the job and shatter the existing order.
I believe his age and charisma are the qualities he brings to doing the job of president. Age, because the job of president is no longer for the old and weary. The president, particularly in a combative environment of hostile media, establishment and cultural warriors requires a large degree of stamina and fortitude. Obama strikes me as the best suited among the contenders to perform this role.
With regards to charisma, for much of the same issues relating to age, the ability of a president to project and explain himself in the media world is a much under-appreciated gift, again especially given the small "c" and large "C" conservative dimensions of the establishment and various centers of power throughout the business and religious world that will seek to undermine any change of direction by a Democratic president.
His race and background help to shatter traditional notions of what a president should look like and where he should come from, professionally and experientially. As the Rogue Progressive notes:
Obama is a member of an older generation who can reach out to younger generations in an authentic way that no one else can. He can break his own generation's tenure-based approach to politics, work and life by not being a presidential apprentice for eight more years. This will move us toward a 21st century way of politics, where politicians will rise and sink quicker because the world moves faster and they will be scrutinized for their performance rather than their resume-building skills.
I'd also add that this is an age where large-scale policy reforms and major legislative battles and achievements will be few and far between. Although there are outstanding concerns, domestically, in regard to inner-city poverty, the criminal justice system, and unequal health care and educational resources, the big ticket items in the liberal planning book have mostly been put in place. The job of the new presidency will be more rhetorical and nuanced, raising awareness of new challenges, inequities and injustices, both here and abroad. But Americans of all stripes will continue to be stymied in their dreams of a new American order as international problems continue to impose themselves on the American government's agenda. In particular, the challenges for the 21st century presidency will be those most hinged to the global economy and struggle for natural resources. The next president will not only have to navigate the already conflict-fused, class and culturally divided world of richer North and poorer South in matters of trade and other economic concerns, they will also have to somehow help bring to a conclusion and begin to reverse the collossal messes created in Iraq, Afghanistan and around the world by the corrosively beligerent and militaristic neo-con cabal.
In short, the new presidency of the 21st century will have to be even more visionary than in previous eras, even while its ability to orchestrate wholesale changes diminishes. We've seen six years of one vision. Obama is the candidate best equipped to create another one.