Because on the majority of substantive issues, they lose the argument. Their anti-government ideology is pretty popular, but their policies are not. Despite a substantial portion of the country self-identifying as conservative, vast majorities of Americans support universal healthcare, progressive tax code that raises the rates on the wealthy, no reductions in Social Security or Medicare, the US military out of Afghanistan and Middle Eastern occupation generally, government regulated environmental protections, a woman's right to choose, etc. etc. Many of those policies aren't enacted or actively supported by the Democratic party either which is hardly a movement of the left, but liberal policy positions on major contentious issues are generally much more popular than conservative policy positions on those same issues.
So because they lose on policy, they (quite effectively) change the nature and frame of the debate and distract with non-issues or like death panels, birtherism, taking guns away etc. where they can garner more widespread support by focusing on straw men or fabrications.
