It's a negative (from the perspective of reproduction) confluence of both social and economic developments.
E.g. the death of the traditional gender roles has inevitably reduced birth rates - for multiple reasons to boot. Because on the one hand, the women has am easier time not to commit and just sleep around, consequently becoming uninteresting to men that would've preferred to make a family... But also because biologically, men are more attracted to demure women, which on average will ultimately remove even more attraction, consequently removing even more likelihood of families being built.
But that's once again only one factor, you got others too... Like stagnant wages, which force younger people too abstain from making a family simply because the financial situation doesn't allow for it. And if it happens anyway, it's more then likely to end in a broken family instead of something positive
Another factor is the availability of choice. Dating apps are available, statistically women all try to get into a relationship with the same 1% of men - who sleep around and cause toxicity all around. The remaining 99% become bitter and consequently... Are even less attractive to women.
Just to be clear, in case someones brain has completely rotten through and interprets any blame into my comment: neither sex is responsible for this. Our society just decided to move on from gender roles, for supposedly economic reasons.
The consequences are felt both for women and men, with both feeling less valued and miserable on average. Which understandably makes them less attractive to the other sex again.
Still not a full list of factors at play btw, there is also the builtup of micro plastics in the men's balls, harming sperm production along with normalization of pornography, reducing the sexual frustration of people and consequently making them less driven to find partners. There is also the influencer industry, purposefully encouraging para social relationships, satisfying the social urges of a lot of people, consequently reducing the likelihood of them seeking out friendships... Reducing the likelihood of meeting other people and thus reducing the likelihood of getting a natural relationship through that.
Third places have also mostly vanished, likely related to multiple of these effects to etc pp