1. Because the tide has swung. There's a political realignment going on. This is the only time in my lifetime that the populace is ready and willing to vote for a liberal for president. Let's give them one, particularly as we happen to have one with cross party appeal.
2. Because he had the courage to oppose the war when he was running for the U.S. Senate in 2003. Political courage and honesty has to count for something.
Of course there are many other reasons, including this woman, but those are the biggest two for me. Any others?