I think the consensus is that it’s mostly as a result of women having greater reproductive choices, greater access to family planning services, and more women choosing to delay having children or choosing to not have children at all, often so they can instead focus on a career.

Edit: I want to point out that what I’m describing is the consensus, as I understand it, of mainstream experts in the US. However, I believe there is evidence that this consensus opinion is not entirely accurate. If I’m not mistaken, surveys indicate that there are a fair number of people who would like to have children but are not because the right circumstances are not present for them to feel secure enough to have children. Many of the people who are not having children would have them if they felt more financially, romantically, and/or emotionally secure. Therefore, it’s possible that it’s not so much that people are choosing not to have children as it is that the necessary conditions for making people feel secure enough to have children are not present for a large number of people.