Expense. For people who don't want to have children at all then no amount of money will persuade them to have one but there are many families who would have 2, 3 or even 4 if they could afford to do so but need to stick at a lower number. Women often need to go back to work full time asap.
Cost of housing, huge difference in cost between a 2 and a 3 bedroom house, and again between a 3 and a 4 bedroom. People having to work more hours to afford a much smaller property than their parents did and in some cases commute from further away. Spending 12 hours out of the house every day will naturally lead to singletons not being as available for dates and meeting people, and couples have less time at home together to make babies.
Many useless men who expect their wives to earn 50% of the income whilst doing 100% of the child rearing. Wives much less likely to want sex whilst having to look after a manchild much less have another actual child.
Families living further apart, people not having a "village", don't know their community or neighbours, no local support with children they have means less likely to have more.
Labour and birth is hell on earth, plenty of women traumatised by births due to lack of staff and awful postnatal care, that they don't want to repeat the experience. Birth injuries, birth trauma. In some severe cases women unable to have more children.
Mental health issues among parents and woeful mental health care making it unwise for them to have more children.
The rise of dating apps and instant gratification of getting several matches gives people of both sexes the illusion of choice. Don't like the way match number 1 dresses, never mind onto the next one and the next and the next. Eventually just leads to people being single for a long time.