It doesn't make sense that religious people are more conservative and secular people are more liberal. It makes more sense the other way around. Why are people like that? Stereotypes.
Religious people and liberals have the same stereotypes. They're both motivated by good, holier than thou, hypocritical, ruled by feelings over logic, etc. People attack them because they're too "good". Secular people and conservatives also have the same stereotypes. They're selfish and evil, prejudiced, ruled by logic over feelings, etc. People attack them because they're too "evil".
Of course, these are stereotypes. They are not true.
People choose religious views and political views that cancel out stereotypes. If you're very religious and very liberal, people think you're pure good. Sojourners is a great example. I read it sometimes for two reasons: 1) it's interesting, and 2) they're gooder than me. Not better, but gooder. You know what I mean. I admire them but I don't want to be like them. They care about people a lot, but they sound like wimps who don't know a thing about the real world. In real life, a lot of religious liberals are pretty tough.
If you're non-religious and conservative, people think you're too evil and don't care about them. The people at The Atheist Conservative are harsh and they don't mince words. They're angry and they don't have to look like hypocrites.
If you're religious and conservative or secular and liberal, the stereotypes cancel each other out. People judge you on your character. People don't think you're heartless and dangerous. People don't think you're self-righteous and wimpy. They think you're normal like them. It moderates your views so you don't sound like a wimp or a brute. Lots of them sound like wimps or brutes anyway.
Also, gay abortion.
There's other reasons: Secular people are rarely Republican because Republicans weave religious language into everything and exclude them. Non-religious people are a minority group and most minorities are liberals because some conservatives hate minorities. Nobody wants to go where people hate them. This is also why most black people are Democrats regardless of religion - some white Republicans still hate black people. Or some Democrat leaders say Republicans do to get votes.
Frankly, religious liberals and secular conservatives make more sense than the other way around. Why? Their arguments don't bore me to tears, and they're both less prejudiced because they're minority groups themselves. I don't know how Jesus or Athe would vote. It makes me sick when I see "Support Jesus! Support (political issue here)!" or "Jesus was a (conservative/liberal/libertarian/anarchist/fascist/socialist)". Also, I don't know how you can be non-religious and have a positive view of human nature like most liberals do. Still, thank God for secular humanists with a positive view of human nature. I'd rather be around a bunch of them than a bunch of Objectivists.
Sorry libertarians, I left you out. You're not a stereotype group on CNN or Fox News.