The reason isn't because it is more difficult or people enjoy having kids less (like the parent). Its because children used to be a way to provide security for your family and community. More hands to help or sent off to earn money younger for example.

Now with smaller family units and less community interaction they represent a risk to security, mainly financially.

“A” reason?

Given the post you’re replying to, it seems you’re implying a specific reason, but what if it’s a different one? How about “I love children but having kids is super expensive”?