There are at least two main reasons why this would be. One is economic and the other has to do with the maturity level of the couple at the time they marry.
First, there are economic reasons. People who have been to college are more likely to have good jobs. They are more likely, therefore, to be financially well-off. When couples are in bad shape financially, there tends to be more stress placed on their relationship. When people go to college and get good jobs, they are more likely to escape these financial pressures. Less stress in a relationship means that the relationship is more likely to last. Therefore, good finances will lead to less divorce for this demographic group.
Second, they tend to get married later and not to have children until they are married. What this means is that both partners come to the marriage at a more mature stage in life and with less “baggage.” The partners have been through much more life by the time they get married and are much more able to deal with things like adversity.
These are the two most common explanations for the lower rate of divorce in this demographic group.