They come in with all these expectations and unrealistic notions an base most of it from religion thinking that is going to save them. Guess again!

In my experience, it is because what they have been taught is not the reality.

Possibly because marriage is expected as the next level for a Christian couple whereas an atheist couple could live out their entire relationship as just partners.

So some people who get married young at 20-25 don't know everything about their partner and realise later in life that they dislike their partner.

Just an idea. I can't prove it or anything.

Christians, especially in the bible belt have a higher divorce rate than atheists per capita last time I checked.

I can point to several reasons:

• Getting married is part of being a Christian. You are forced into it, many times before having the maturity to choose a proper partner.
• Churches teach the wrong standards to pick a partner. Usually it is all about public character and nothing about interpersonal relationship.
• Abstinence plays a big role. A lot of Christians get married just to have sex, way faster than they should (guilty as charged, sadly). That tends to happen in more conservative settings, of course.
• Again, in literalist churches marriage starts with 3 individuals. But at some point one of the partners realize that God is not pulling his weight and either take advantage of that or counting too much on someone that just isn't there.
• Sometimes one spouse (or both, but mostly the woman) didn't get much of a say in it, and later finds one's voice and ability to act on one's will.

And the list goes on and on, not to mention the reasons not particular to religious couples.

Because a christian can wake up one day and realize their partner is batshit.