Six years ago a Christian polling group looked into this very question. Since much of the declines in US Christianity are among the younger generations, that's what they focused on.
https://www.barna.com/research/six-reasons-young-christians-leave-church/
Reason #1 – Churches seem overprotective.
Reason #2 – Teens’ and twentysomethings’ experience of Christianity is shallow.
Reason #3 – Churches come across as antagonistic to science.
Reason #4 – Young Christians’ church experiences related to sexuality are often simplistic, judgmental.
Reason #5 – They wrestle with the exclusive nature of Christianity.
Reason #6 – The church feels unfriendly to those who doubt.
Also, if you look at the data in the OP, it's the non-conservative areas of Christianity that are declining the most, while conservative evangelicals are roughly staying level. Given the rather "extreme" positions evangelicals take on things like LGBTQ issues, science, and cultural diversity, I wonder if we'll reach a point where American Christianity is mostly composed of right-wing fundamentalists. And then I wonder....given the above survey.....if that will only accelerate the declines we're seeing today.