I think two of the more uncomfortable reasons that don't get discussed might be:
1. The allure/requirements of the modern lifestyle (both people working then limited leisure time) reduce the 'space' for childrearing
2. Depression, a negative outlook about the future, and awareness of the environmental impacts of more people mean adults consciously abstain from having children.
People are depressed on a social scale. Few people have a real passion for life and reason to live. Even our "leaders" are milquetoast technocratic caretakers -- not very inspiring.
People generally follow the crowd, and the crowd is busy with hedonistic pursuits.
3. Religion and Culture - atheism has grown rapidly among western cultures over the past few decades. It is possible that western atheists have made a personal choice in service of their individual lives here on this earth, while someone who is guided more by religion or ideology is making a decision for what they view as a higher calling or purpose. This hypothesis is supported by the fact that first generation immigrants to the west tend to have higher birthdates vs those who were born in the west, as well by how common it is see modern religious and social conservatives in the west push for "tradlife."