No, it's not because so many people already hate her. It's not because she's married to Bill. And no, it's not because she's a woman.
It can be explained in these two paragraphs of insightful truth from David Mixner:
Today, the Clintons run the political machine trying to save the status quo in the Democratic Party. Their fundraising operation is notorious for its ruthlessness and elitism. Their circle of advisors and friends are tough and aggressive with anyone who refuses to pledge allegiance. They are surrounded by money collectors like Terry McAuliffe who shakedown donors with warnings that they will be punished if they give to another candidate. Senator Clinton's position on the Iraq War is by far the most calculated of any candidate. And on so many other issues, her positions are measured and break no new ground. Each appearance is predictable and perfectly arranged. Whether by necessity or choice, the spontaneity, exuberance and hope we saw in both of the Clintons in 1992 is gone.
In many ways, Senator Barack Obama is today's Bill Clinton. Like Clinton in 1992, he is packing arenas with young voters, campaigning in shirtsleeves, and calling America to believe in a new generation of politics. His candidacy stands in stark contrast to the safe predictable status quo Clinton campaign. Unlike Senator Clinton, he understood the consequences of invading Iraq and refused to support the war from day one. Like President Clinton in 1992, he is mobilizing thousands of cynical and disenfranchised voters and welcoming them back into the Democratic Party.
He goes on about Edwards, too. Read the whole thing.
Oh, and someone tell the mainstream media.