If marital age is a factor in many of these divorces, then I would say it's because they have never learned to live life on their own before they got married. When people get married at a very young age, they have not fully developed cognitively (Science has shown that cognitive development continues into the early 20s I believe.) and quite possibly emotionally. These people then continue to grow and develop, and if they don't grow in the same direction or at the same rate, then they might decide that their differences are too great to stay together.
There does seem to be a prevalence of conservative Christian couples getting married at younger ages than the general population. I actually happen to know several people who got married right out of high school and sadly, probably more than half of these relationships ended in divorce. Even many of my friends who were married before or just after graduating from college were divorced less than 2 years later.
I agree with the "lack of maturity" as a reason for why this happens, as well as a general lack of experience. Finances are probably another factor that could be taken into account. Financial stress is a leading cause of marital strife at any age, but imagine how youth and inexperience could compound this, especially when many parents cease to provide an "out" once their child has taken a major step into adulthood. Other people go into debt just to get married, paying for the wedding, rings, honeymoon, etc. Several of my friends actually moved-in with one set of their parents, which in every case was more emotionally difficult than financially helpful.
There are just endless reasons why the divorce rate is high among younger people. I'm not sure that being a conservative Christian has anything to do with it, except maybe to say that there is a larger number of conservative Christians who are getting married at younger ages.
I will start this answer by assuming that you are right -- that being unready to live on their own actually is a major cause of divorce. I do not agree with this, but we will start with this assumption.
If you are right, this problem stems from a lack of maturity. People who are not ready to live on their own presumably are not yet secure in who they are and who they want to be. They do not yet love themselves enough to be happy while alone.
If you do not love yourself enough to be alone, then you cannot possibly enter into a committed relationship with another person. You will not be ready to commit to another person because you are not yet committed to yourself.
So if your hypothesis is true, it is because these couples are not mature enough to commit to each other. They have not yet accepted who they are as individuals so they cannot really commit to one another.