For many if not most people, the word "atheist" carries as much emotional baggage as "communist" or "anarchist," and that's why a lot of people who are atheists aren't comfortable expressly identifying as such. 

I don't think it has anything to do with political ideology. Christians are not exclusively conservative, and atheists are not exclusively liberal. For me being an atheist is not something to necessarily be proud of, any more than being right-handed or liking pizza. I just determined at an early age that I just wasn't getting the hang of the whole "god" thing. It was as though my antenna just didn't pick up that frequency. I'm content to live in the real world. I don't need to be beholden to an invisible world of angels and demons and gods and prophecies to have meaning and purpose in my life. And I suspect a lot of other people feel the same way, if they aren't so up-front about it.