There are lots of potential reasons.
1. Too low a bar of what we call a 'Christian.'
In a lot of churches, the preacher doesn't explain what sin is, that Jesus died on the cross, that he rose from the dead. Whether he does or does not, he then asks people to raise their hand to 'give your life to Christ' or to 'ask Jesus into your heart.' So you have some people going through a ritual of repeating a prayer without having repented, or in some cases, without having the Gospel explained to them. Then, the preacher may declare them Christians and they leave thinking they are born-again Christians, rarely go to church, and are in the pool of potential people to fill out marriage surveys.
2. A lack of teaching on marriage and divorce.
If you talk about that in church, you'll step on people's toes. They might go to the other church down the street or not give money. It's also uncomfortable to talk about the subject since so many divorced and remarried people come to church. So some preachers don't pay much attention to it.
3. Not believing the scriptures.
Since other preachers don't talk about it, maybe it isn't that important, the preacher reasons. He thinks, maybe those verses don't mean what they say. Maybe they really mean what all these divorced and remarried people say they mean.
4. Not practicing church discipline and restoration of individuals.
If a couple want to get a divorce, sometimes the church doesn't approach them to help them. Some couples find there is no support from older couples to teach them how to get through difficult times until they start talking about divorce and the church gets involved. Sometimes a divorced person-- who should be reconciling his or her old relationship-- wants to remarry and no one in the church checks up on him/her to help keep him/her from the sin.
A little leaven leavens the whole lump.
Not dealing with fornication and adultery in the church can lead to more of it spreading. I've read that there is research that shows that brides not being virgins at marriage greatly increases their chances of divorce. Adultery, clearly, can hurt marriages and can put a marriage at much greater risk for divorce.
5. Tolerance of leaders divorcing and remarrying wrongly.
Sometimes leaders in an overseership role will get divorced and not even take time away from ministry. An overseer is to rule his own house well. Some leaders will divorce and remarry adulterously and remain in ministry.
6. Acceptance of Feminism and Other Worldly Philosophies Lead to Weak Men and Rebellious Women.
Many men accept the prevailing philosophy of Feminism that has crept into church teaching and don't act as men should in their own home. Many of them were born a generation or two too late to be surrounded by male role models who knew how to lead their own homes well.
Many women also accept the idea of Feminism, which can lead to a rebellious, disrespectful attitude toward their husbands. Feminism and other worldly philosophies tell women they can leave their husbands if they are not happy, or if they can find some small fault with their husbands. The lack of obedience to the command to respect/reverence the husband can lead to discontentedness and divorce. Men who accept the prevailing philosophies may also leave for what they consider to be greener pastures.
Men who accept Feminist philosophy, whether they realize they accept it or not, who buy into the myth that women just want a sensitive guy and that the way to make women happy is to do the things for them that they say they want may find themselves acting like boys, treating their wives like their mothers. An overly 'sensitive' whiny, obedient husband who doesn't stand up for what's right, who makes her make all the decisions is unattractive, even repulsive to some women.
7. Ignoring the commands for husbands and wives in scripture.
I suspect much if not most marital problems come from not obeying passages like Ephesians 5. If wives would submit to their husbands and respect them, there would be a lot less arguing in marriage. If you reverence someone, you don't yell at that person or call him names. If husbands would love their wives as Christ loved the church, they wouldn't be abusive toward their wives or take them for granted. If men honored their wives as the weaker vessel, it would show up in the way they talk to them, and the way they take responsibility around the home instead of leaving things up to the wife.
---Good news.
I've read that couples who regularly pray together have divorce rates far under 1%! If someone knows the source for that, please post it. I've heard it from a lot of places.