Because Obama blew it over healthcare:

In the days after John Edwards’s withdrawal from the Democratic race, the political world expected his endorsement of Barack Obama would be forthcoming tout de suite. The neo-populist and the hopemonger had spent months tag-teaming Hillary Clinton, pillorying her as a creature of the status quo, not a champion of the kind of “big change” they both deem essential. So appalled was Edwards at Clinton’s gaudy corporatism—her defense of the role of lobbyists, her suckling at the teats of the pharmaceutical and defense industries—that he’d essentially called her corrupt. And then, not least, there were the sentiments of his wife. “Elizabeth hasn’t always been crazy about Mrs. Clinton” is how an Edwards insider puts it; a less delicate member of HRC’s circle says, “Elizabeth hates her guts.”
But now two months have passed since Edwards dropped out—tempus fugit!—and still no endorsement. Why? According to a Democratic strategist unaligned with any campaign but with knowledge of the situation gleaned from all three camps, the answer is simple: Obama blew it. Speaking to Edwards on the day he exited the race, Obama came across as glib and aloof. His response to Edwards’s imprecations that he make poverty a central part of his agenda was shallow, perfunctory, pat. Clinton, by contrast, engaged Edwards in a lengthy policy discussion. Her affect was solicitous and respectful. When Clinton met Edwards face-to-face in North Carolina ten days later, her approach continued to impress; she even made headway with Elizabeth. Whereas in his Edwards sit-down, Obama dug himself in deeper, getting into a fight with Elizabeth about health care, insisting that his plan is universal (a position she considers a crock), high-handedly criticizing Clinton’s plan (and by extension Edwards’s) for its insurance mandate.

With diplomate skills like this, his one on one talks with dictators seem even more scary now.
(via)