Because they hate America.
I don't think it'd so much that as it's a combination of living in a 9-10 world, hatred of George Bush and the political right, and a temper tentrum realizing that they have lost power and are not going to get it back anytime soon. They demand the America always live up to their ideals, and when we don't they go into two year old mode.
(ok ok I know I'm putting a positive spin on them)
And I'm forced to mention that large numbers of the left have gone (IMO) completely insane, stark raving mad.