I used to be Mormon, I think Mormon culture is the main reason. This article seems to describe the reasons well: https://religionnews.com/2017/06/20/10-reasons-mormons-domin...
I learned this weekend this geographic designation for local congregations is shared among the Amish, where congregation (to my understanding and happy to be corrected) are between 20-40 families.
https://religionnews.com/category/faith/unaffiliated-atheism...