Previously people who bore kids but didn’t plan on it were peer pressured by society to carry their water and take responsibility and support them to some degree. Divorced parents faced significant pushback.
These days, at least it seems, without this social pressure people feel free to leave more readily before giving raising their family a greater priority. The priority is their convenience and not the new generation. Also people just don’t apply this social pressure on their peers so much, so there is less consideration when seeking divorce or plain abandonment.
One interesting thing about the ease of marriage divorce in my mind is the incongruence of the entry/exit.
Want to get married? Pay a small fee at the courthouse and get a license in a day or so. Very easy, very low cost.
Want to get divorced? The process can take over a year, cost tens of thousands or more, involve court appearances, and dividing property.
Granted both of these actions are easier or less restrictive than they were in the past. Although the lawyer fees seem to be higher, at least when considering real wages have decreased since the 70s.
With the prevalence of divorce, you would think that prenups could be made a part of the marriage license so that lawyer costs, court costs, etc could be minimized. It might even cause some people to reconsider a bad marriage. You could go farther with premarital counseling if this is really a concern.
I don't know of those are really good ideas or not. Personally, I think the government should get out of the marriage business altogether. With the wide variety of living arrangements and expectations for what a marriage is, I think it might work better for people to enter into private marriage contracts.
On the social pressures and unwanted kids part, I wonder how mate selection plays into this. Sure, there was free love in the 70s, but I still imagine that premarital sex was not as common as it has become today. I wonder how women's expectations of men have changed during this time, especially for when to allow for sex. I also wonder how the expectations men (and society) have of women have changed, especially in terms of work/home/financial responsibilities and contraception/abortion options.
These days, at least it seems, without this social pressure people feel free to leave more readily before giving raising their family a greater priority. The priority is their convenience and not the new generation. Also people just don’t apply this social pressure on their peers so much, so there is less consideration when seeking divorce or plain abandonment.