Apparently because he blew it after Edwards' withdrawal from the race.
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.
I suspected something like this happened. Back in the salad days of this primary race, I wrote about how Edwards and Obama's messages overlapped to such a degree that they would fit naturally on the same ticket. Their shared rhetoric (and their shared attacks on Clinton) led most to assume Edwards would endorse the Illinois Senator if he endorsed anyone.
Now we know why Edwards' endorsement is still an open question.