I think this is def part of it. Trump was not doing well in 2016 at all until the final debate when he cornered Clinton into a (legitimate) strong defense of her pro-choice position.

All the "moderate" Christians who couldn't stomach Trump before suddenly had no choice.

Essentially all Christian denominations + Mormons think abortion is murder. How can a candidate win a majority in a society where a plurality identifies as Christian and therefore probably takes that position?

Secularization of the majority, and the liberal culutral values that go with it just alienates these people more and more around abortion, gay rights, and most markedly, trans issues.

Although the devoutly religious are becoming more of a minority, they are far more homogeneously aligned on these core issues, and therefore easier to cohere around a "right wing" electoral block even when they do not think "right wing" around economic and political / international issues. They're willing to tolerate Trump on a whole pile of things as long as they feel he's accomplishing their "moral" goals -- and so far he mostly is.