It's over and everybody is asking why. Taylor Marsh at Pajamas media sums it up pretty well. How about blaming it on a vast left-wing conspiracy?
Practically speaking, her history on Iraq and Obama's superior political skills were the overriding factors. Hillary's idiotic "if I'd known then what I know now" defense of her Iraq vote was one of the lamest ever but she was getting away with it until CNN's Wolf Blitzer blasted her on it during the California debate:
Obama easily won this argument (that he's the only Democrat candidate suited to go up against the GOP based on his anti-war tact), which of course totally ignores the strategic reasons we went into Iraq or the aftermath of a precipitous withdrawal.
Fortunately for Obama the Iraq issue was in the forefront early in the primary when he needed it, which helped him gain the needed momentum while Hillary was dealing with Norman Hsu and her teary-eyed moment in New Hampshire. When the bad news stopped flowing from Baghdad and Reverend Wright took center stage he already had a commanding lead, allowing him to weather a late surge that came despite the revelation of Hillary's bald-face lie about dodging sniper fire in Tuzla. At that point many people were voting against Barack instead of for Hillary, but thanks to the media's early help it didn't matter.
Indeed, Blitzer's question in Los Angeles represented a sort of turning point for the media, a passing of the baton so to speak, indicating they had a new winner and didn't need to run interference for the Clinton machine anymore. Long time supporters used to favorable coverage were shocked and appalled and began to sound more like children as the days passed.
But whatever the case and regardless of the outcome in the general, witnessing the demise of the Clintons' bid to capture a third term in the White House represents a good day for America.