I think it has to do with how the parties appeal to people.
Conservatives have become good at appealing to fear-- whether it's fear of negroes Muslims, government, socialism, the UN, taxes on rich people, or illegal immigrants.
Liberals just suck at peddling fear. Note how even after the huge BP spill, most people still support increased offshore drilling. Even after a huge economic collapse caused by the mechanisms to make the rich richer, most Americans don't think wealth concentration is a problem. Even though everybody is at risk of losing their job and therefore health insurance, liberals can't convince America that that is something to be afraid of that universal healthcare would protect them from.