This study, released today, has significant finding about the decline of Christian affiliation but does not provide explanations for the declines discovered in their surveys. I would suggestion the following reasons:
Personal rather than political reasons
The breakdown of families, which normally have held people in churches
Church-life feeling more hurtful than helpful
Social norms that caused people to say they were religious in the past are fading. People are more comfortable admitting they are unaffiliated.
America’s Changing Religious Landscape | Pew Research Center.