Probably because certain pagan thinkers like Plato and Aristotle were very compatible with monotheistic immanentism, while polytheistic paganism is very fractionalized (much like Christianity today) usually by race, and due to Romes weakening caused by racial and gender issues lead to people running to Christianity (fresh faces bring fresher pastures) and due to the massive force conjured throughout the roman empire was able to spread to the rest of Europe
