Two things come to mind.

One, Gore's loss in 2000 may have soured some as to the viability and wisdom of third party campaigns in our political system.

Two, this realization has made many understand that if we are to have a real voice in American politics and policy we need to do it in the framework of the Democratic party and what we have to do is effect change within the party to get our voices heard. Howard Dean both clarified that understanding and provided a rallying point.

God bless him. And I mean that literally.