Americans who are unaffiliated with any particular religion have seen the greatest growth in numbers as a result of changes in affiliation (Pew Report),
The answer to why
loss of faith seems to be accelerating, at least among people in some cultures, could be evolution.
Step into the tall grass for a discussion of that idea.
The near universal observation of religious and cult practices by our ancient ancestors, suggests that being religions may have been pro-survival for humans throughout most of our history. There is considerable disagreement among scientists on the psychology of religion and its evolution. However, human history suggests that throughout humanity's evolution, religion has played a central role in organizing groups of us to work and live together, and it is joint coordinated human action that accounts for mankind's most visible achievements (and crimes and failures).
For most of humanity's existence, religion has provided a way of organizing what people knew about the world where they lived. People needed an explanation for the dangerous places they inhabited and know how to live together; religion told them how to be born, how to mate, live and die and gave them stories to explain the world where it all happened. The stories religion provided were often wrong or even impossible. Meanwhile, the conservative and static nature of faith allowed mankind to grow slowly in a slow paced world. Everyone professed their religions to be infallible, and so they were. Until just the past few thousand years there was nothing better to answer humanity's questions about the universe. Then science came along and gave humans a better way to understand their world and survive together.
So, it is this non-scientist's speculation that religion probably had selective advantage for humans, for as long as there have been humans, but science has come far enough along that, now, losing faith may have selective advantage, instead.
More importantly, unlike evolution conventionally, lack of faith can be passed on socially. My own adopted daughters do not share my genetic material, but these incredibly good women do appear to share my worldview in this regard, though it was always entirely their choice.
Do not mistake this idle hypothesis to suggest that atheists are somehow more evolved or better than religious people today. That would be nonsense. Nevertheless, science indisputably provides better answers for how the universe operates than any church. Ask Galileo Galilei. The looming crisis of climate change illustrates that getting answers right with science is important for a world spanning industrial species.
Many human practices once common and widespread have faded away socially as the challenges of human existence have evolved. Some of these, human sacrifice comes to mind, seem particularly abhorrent. Chattel slavery comes close to that. In all cases religion supported or even outright required such things. As a species, it is possible that human survival, long term, which once might have depended upon religiousness, may now be evolving to depend upon abandoning faith as a way of answering important questions.