The book suggests that the defining problem driving out most people who leave is … just how American life works in the 21st century. Contemporary America simply isn’t set up to promote mutuality, care, or common life. Rather, it is designed to maximize individual accomplishment as defined by professional and financial success. Such a system leaves precious little time or energy for forms of community that don’t contribute to one’s own professional life or, as one ages, the professional prospects of one’s children. Workism reigns in America, and because of it, community in America, religious community included, is a math problem that doesn’t add up.
It's about perceived values. Attending church in the former society was of personal value because it raised your connection and status in society. In this work culture it does not, unless of course you create a new church of work that does provide some sort of value and status to your professional life.
This thought isn't entirely fleshed out but there I am quite sure that this gap can be bridged with some creativity.
TBH I don't think it's any one thing. I was raised Methodist and we basically stopped going because my parents didn't care enough to go. It's not like there was any status associated with our tiny church, or that they liked/disliked the people there; it's just that no one wanted to spend an hour in church every Sunday.
I think it varies from person to person, though. Some people stop because of abuse or disgust with the church. Some people have a sudden moment where they realize it's all fake. Some people just don't care.
But the point here is that this is because previously they got something out of it, because society was different. Whereas now there is no perceived value other than the virtue of being a churchgoer, which is not really seen as one in wider society. The value beyond this that it previously created has gone.