Either that or the sheer terror that the US is slowly becoming less religious(30% of Americans are openly non-religious) in just a few decades and a lot more people then that are nominally non-religious(Professing christian beliefs but not being terribly observant). To the evangelical/fundamentalist types who BELIEVE a Christian Theocracy would be a great idea(nevermind the fact it wouldn't, even for them) I imagine it's pretty terrifying. It goes without saying that the venn diagram between US fundamentalists/evangelicals and fucking morons is pretty close to a circle.The mindset: "Anything that I disagree with must not exist. Land of the free!"
There's also the factor a lot of such folks see Atheism as synonymous with Communism, which is even dumber but there we are.