There are lots, but the simplest one is that children come from marriage, and statistically having a father and a mother is clearly the best outcome for kids. Unmarried couples are far less likely to stay together. Patterns of promiscuity pre-marriage, plus a culture that doesn't value faithfulness (i.e., porn, no-fault divorce, etc) surrounding a marriage make it harder to stay together long term. Divorce teaches children to fear commitment, perpetuating the problem.

This podcast is heckin' good on the topic: https://podcasts.apple.com/us/podcast/katy-faust-our-childrens-forgotten-rights/id1547129171?i=1000608655471