Mix of financial and personal reasons.
Lots of people have never wanted any children and chose a childfree life.
Others are not able to have children of their own or have serious health problems that rule out raising any kids.
Some decided they they were not financially able to support a family that included children so either decided not to have children or not to have sexual relationships.
Most of those who do have 1-3 children planned how many children they would have, deciding to have children or add to their families only when they were secure in their careers and finances.
The easy availability of safe legal birth control has helped those who wanted to delay having children, limit their unplanned pregnancy risks or avoid having any biological children to do so.
Leading to average family sizes in most of England getting smaller compared with what they were 50 years ago.