There are many inputs but I'd wager it's some combination of cultural and physical phenomena:
In the west, kids are taken for granted and regarded as an impediment to career and self. Women are subtly taught that their impulse to be a mother is the result of cultural brainwashing by men that seek to control their bodies.
Hormonal birth control. Among sexually active women that anticipate having children later so haven't sterilized themselves, most are on some form of hormonal birth control.1 This is largely regarded as harmless relative to the convenience but I suspect it dulls the psychological impulse to have children in addition to biologically preventing it.
Men and women aren't coupling like they used to.2 Women are subtly taught that high quality men want an identical twin - an ambitious, aggressive, career driven, capitalist - and the men that don't want that are abusers. They are also taught that orienting yourself toward family formation means subjugating yourself to men.
Chronic illness. If an illness isn't directly impacting fertility, most people aren't going to want to create dependents when they feel like crap.
Economic conditions. When people aren't doing well themselves or are uncertain about their financial future, they are going to be more reluctant about starting a family. Further, the career landscape is much more dynamic and unstable than it once was.
Cultural and economic feedback loops. When people have less children, child services lack scale and become more expensive, which causes people to have less children, and so on. When people have less children, fewer people you know have children and there are fewer public advocates for having children, which causes people to have less children, and so on.
Footnotes