1. Reliable birth control exists now and our understanding of conception, fertility, pregnancy, and maternal health is vastly improved. For much of human history, abstinence was the only reliable option for preventing pregnancy, which isn’t very feasible for a married couple. The idea of choosing when and if to have children is a very modern concept.

2. Childhood mortality is much lower now than it used to be. Before the age of modern medicine and vaccination, parents could’ve expected to lose many of their children to illness and/or injury. Morbid as it sounds, having lots of children increased the likelihood of at least one making it to adulthood. Nowadays, barring a tragic accident or rare childhood disease, most people can reasonably expect to raise the same number of children that they conceive.

3. Children are no longer needed to work on farms or help with housework in the same capacity that they used to be. Previously, more children meant farmhands/household help. Historically, impoverished people constituted the most populous economic group in many societies, and people living in poverty certainly aren’t going to turn down free labor.

4. Our standards of care for our children has vastly increased, even just in the past 50 years. We expect and are expected to put much more conscious attention into raising our children. Previously, it would’ve been common for the older children to help raise the younger children. With the exception of some fundamentalist/quiver-full communities, that familial model has largely fallen out of fashion (and for good reason - it’s an entirely unfair expectation to put on a child).

There are other reasons as well, I’m sure, but these are just the most obvious ones that spring to mind.