I think the 1910 figure might have more to do with practical concerns. When you have no car and live on a farm far from anything, as a huge number of people did back then, you can't regularly get to church even if you wanted to. So I wouldn't interpret that number as a clean measure of religiosity.

Go for a drive in new england sometime and count the churches in the sticks. There's usually 2-5 congregations per tiny little hamlet, most of them built in the mid-1800s or earlier; most of them now defunct or dying. Sometimes you get 3 schisms of congregationalists in a row!

"build a church and school within reasonable walking distance" was rule 1 of new towns for a good long while.

Could some of that be due to the shift from a rural to urban society? If getting to church requires taking a horse and buggy 10 miles down dirt roads, then most weeks you might have a DIY religious observance at home on the farm instead. If you live in a town where you can walk to a neighborhood church, you're probably more likely to make it a regular habit.

It's kind of the other way around: what was so weird and new about the 50s that performative church attendance was so high?

And i can think of ~three answers: post-war trauma, a population bubble, and a percieved need in the white middle class for social discrimination and "order" against internally, integration and externally, "the godless commies". (see: HUAC, adding "under god" to the pledge)

I figure that the 50s were an anomaly, not the other way around.