This story hit me pretty hard. While I was in medical school, my wife's family, especially her mother, routinely encouraged her to divorce me.
We had two small kids, and I had gone on job interviews and seen the offers I was getting without a graduate degree, despite a BS in physics and 7 years of experience. It was obvious to everyone that medical school was the right course of action in the long term but it would be hard.
I didn't realize it would get harder when her family started jumping on any opportunity to point out my "temporary financial embarrassment". On a day-to-day basis, she was working 40 hours a week and while med school was paid for, and I had a small stipend, her paycheck was better. And cognitively, she understood that. But her brothers and mother, and eventually her father told her to "get rid of him".
We stuck it out. They wonder why we don't fly across country to visit them much...
My father used to quote someone that "the best a man can do for his kids are to love their mother."
And: I feel sorry that this can actually happen to people.
I wonder if the other side of the coin isn't that wives tend to respect their husbands less when these earn less than they do. It could be because it violates a social norm caused by society's gender role expectations.
Research found that even employed men who earn a bit less than their wives are affected by higher divorce rate.
So we can cast these men as insecure and emasculated, or it could be a mix of both men and women's reaction to the situation, along with many other variables that you've hinted at.