because they thought that the desease came from crows
Based on the map I've attached, I'd say that Christianity spread to
most of the Roman Empire by 476 AD. If you look at the map, you will see that before 325, there were some places in Spain and Italy mostly where Christianity existed, however, after that year, it only continued spreading throughout Europe, which mostly belonged to the Roman Empire at the time.
I believe it is because that is where jobs were, and where family and friends were.