1) Decline of stable men.
2) Decline of stable women.
(For points 1 and 2 I mean in all things - emotional maturity, actual maturity, financial security, compatibility, cultural issues, psychological factors)
3) Financial stability.
4) Examples your parents set end up giving you so much burnout you can't imagine having your own children and going through it.
5) Effectively spending all of your time and money 'raising' your parents in their old age.
6) Repressive cultures.
7) Fertility issues, sex-ed awareness, stigmas (women's reproductive and sexual health remain underfunded, understudied and millions of women are developing inflammatory reproductive issues which vastly affect their health and psychological well being. society's answer to this is BC as a medicine which isn't helpful due to numerous side effects including not having babies.)
8 ) we spent decades telling young potential parents to wait for 'marriage' only to have kids when married (so they enter into clueless marriages, eventual broken marriages too), to abort or give up their kids due to stigma.. and guess what? Now less young people are choosing to have kids before marriage.. but are also choosing not to marry or have kids at all.
There is no one chief factor.