I say because, so the evidence shows, if a (heterosexual) man is to get married and stay married he has to offer a lot of money to the woman.
So, you're probably familiar with the fact that in the high-income strata of society, traditional marriage and child-rearing patterns have largely been maintained, while lower down the social ladder we find more single parent families, divorced or never married parents and so on. You might have treated this as a fact about upper middle-class 'values' or deferment of gratification or what have you. But this is a more simple explanation:
"the distribution of the share of income earned by the wife exhibits a sharp drop to the right of 1, where the wife’s income exceeds the husband’s income. We argue that this pattern is best explained by gender identity norms, which induce an aversion to a situation where the wife earns more than her husband. We present evidence that this aversion also impacts marriage formation, the wife’s labor force participation, the wife’s income conditional on working, marriage satisfaction, likelihood of divorce, and the division of home production. Within marriage markets, when a randomly chosen woman becomes more likely to earn more than a randomly chosen man, marriage rates decline. In couples where the wife’s potential income is likely to exceed the husband’s, the wife is less likely to be in the labor force and earns less than her potential if she does work. In couples where the wife earns more than the husband, the wife spends more time on household chores; moreover, those couples are less satisfied with their marriage and are more likely to divorce. These patterns hold both cross-sectionally and within couples over time."
(That "gender identity norm" is just the norm that a woman won't put up with a man unless he's bringing more money than her to the party: i.e. it's just another way of saying that men must be pretty horrible.)
Now, when you take into account the fact that the difference between men and women’s earnings is much larger at higher household incomes than at lower incomes, you can see why poor men don't get married: unlike rich men, they just don't have the massive advantage of earning power over women which is necessary to persuade women to stick with them. (All at the link above.)
This fact about the inherently off-putting nature of men explains divorce as well: women are more likely to instigate divorce proceedings (it's about 70/30), even though they are likely to end up worse off as a result (average 30% drop in income).
You might want to think about Scandinavian countries in this context as well, famed as they are for sexual equality, low marriage rates and high divorce rates. "The equalising of income which has occurred among social classes and between the sexes in the 20th century has contributed to making divorce a viable alternative, not just for the elite," says Glenn Sandström of the University of Umeå. Yup, even those lovely Scandinavian men are horrible really.
What does this mean in the long run? Now that women are entering the highest earning professions at rates similar to men (basically the same for the Bar, for example) or even ahead of men (over 60% of trainee solicitors and new solicitors are women, and already most GPs are women), something has to give: all of these female professionals are going to have to abdicate larger and larger shares of their earning potential, or marriage rates are going to come down, or the fact that men are horrible is going to have to change. Let's assume that men won't change. What next?
Of course, the normal way in which a woman sacrifices a large proportion of her earning potential is as a result of having children. But across the world, and particularly in rich countries, people aren't having as many children as they would like to have, principally because of money worries and the cost of housing, as well as the infertility of older women.
And in the very long run? "Russians regard 2.3 children as ideal; Spaniards favour 2.4; Greeks think 2.6 best. In all three, people reckon that they will end up with 1.7 children on average. Because the replacement fertility rate is about 2.1, the difference between the ideal and expected number of children in these countries is the difference between healthy natural population growth and natural decline." In the very long run, we are all dead. And I mean all. Still, that will solve the problem of what to do with those horrible men.