@j in terms of European folklore traditions, I'd think that the written records would be one reason. Like we have many works from Ancient Greece that incorporate the mythology (Homer, Sappho, Sophocles, etc.) whereas we don't have comparable amounts of work from Norse or Celtic cultures.

Also many early Christian works (including the New Testament) were written in Greek & Latin, so the languages were widely studied in the past millennia.

if you wanted a real answer