One convincing explanation is that many millennials have watched their parents try to juggle two careers and saw how stressful it was. Others saw their families fall apart with divorce or single parenthood. When the family falls apart, a whole world unravels.
I can say, anecdotally, that there is truth to this.
My grandparents on both sides of my family were divorced. On one side, it was particularly bitter and the strain of it all arguably led to my grandfather's death, among a host of other problems. The other wasn't quite so traumatic.
My parents saw what happened and didn't want that to happen to me and my siblings. They're still together, as are the vast majority of my many aunts and uncles, who went on to lifelong marriages.