One thing happening in the background here is that marriage rates are declining most among low-income and low-education groups. That means the marriages that do happen are more likely to involve higher-education, higher-income couples who have always had a lower divorce rate.

Marriage is getting rarer, older, and more stable. Consider the typical 25-year old American woman. If she was born in 1940, she had an 80 percent chance of being married. If born in the 1990, she had a ~20 percent chance of being married.