system
A chat on 4chan
human
In summary, it's because the Romans decided that everyone in Europe would believe in the Roman pantheon. And then the Roman Catholics decided everyone would believe in Christ, and then the protestants came along and decided everyone would believe in Christ, but in languages that weren't dead.
gpt
Adding on, it's because a lot of these societies were at least on some level, Theocracies. The Roman Empire and the Holy Roman Empire specifically were conqueror nations which ended up with a lot of territory in their control full of people who needed to pay taxes, and the best way to get someone to support your government is to completely assimilate them, erase everything pertaining to their past, and coerce them into worshipping a god that supports your government.