David Gelernter offers up one explanation:
…Where does the asymmetry come from? American conservatives tend to be Christians or Jews. Liberals tend to be atheists or agnostics. (Yes, there are exceptions—to nearly everything, always; but that doesn’t mean we can stop thinking.) Almost all human beings need religion, as subway-riders need overhead grab bars. The religious impulse strikes conservatives and liberals alike. But conservatives usually practice the religion of their parents and ancestors; liberals have mostly shed their Judaism or Christianity, and politics fills the obvious spiritual gap. You might make football, rock music, or hard science your chosen faith. Some people do. But politics, with its underlying principles and striking public ceremonies, is the obvious religion substitute.
Hence the gross asymmetry of modern politics. For most conservatives, politics is just politics. For most liberals, politics is their faith, in default of any other; it is the basis of their moral life…
Makes sense – so, when we attack their idea of government, we are attacking what they hold to be sacred…and they will come after us heretics.