Liberalism collapsed because:
It became an elite demographic rather than a grassroots movement.
It lost interest in the economic issues that had made it relevant.
It came to prefer fake icons like Obama and the Clintons rather than real programs.
It dissed the folks it should have been converting. The working class just became Rush Limbaugh listeners not worth the time of day. Liberals condemned instead of connecting.
It made the federal the enemy of the local rather then an ally as it had been under the New Deal and Great Society. Instead of sharing decisions and money with the states and cities as the New Deal did, Washington became the boss who knew it all.