Prominent and powerful Flat Earthers have typically been motivated by the Bible. For example, this guy:
https://en.wikipedia.org/wiki/Wilbur_Glenn_VolivaRowbotham was a super Christian and was motivated by proving the Bible true by proving Flat Earth true.
Christianity and Flat Earth theory are pretty much intrinsically connected, athiest Flat Earthers are probably in the minority.
If you're an athiest and if you have any formal education at all, you likely believe in the natural, emergentist structure of reality (ie. quarks < atoms < molecules < proteins < cells < complex life < psychology < sociology < ...) and of the cosmos (moons < planets < stars < solar neighborhoods < galaxies < galaxy clusters < supercluster filaments)
Flat Earth doesn't exactly fit in with the emergentist view of the universe at all.