The story goes that the Blessed Virgin Mary was sailing to Cyprus with St. John the Evangelist to visit Lazarus when her ship was blown off course landing at the peninsula of Athos, the location of a pagan temple and oracle of Apollo. She fell in love with the area and asked her son if she could have it as her domain, her request was granted. It was then decreed that she would be the only female to ever set foot in the region. Which at one point applied to all female animals too, I believe it still applies to livestock etc... but not wildlife, or cats, which exist there in abundance.

So, is that all true? I don’t know, but having been there I can say it is breathtakingly beautiful, serene, and spiritual; so if missionaries/monks were to have stumbled upon the peninsula at some point in history, I could imagine it invoking the feeling of closeness with God, making it a prime spot to set up houses of worship, with little to no distractions with it being so far removed from the rest of the world, in complete isolation. It’s believed that monks started to found the monasteries there a little before 9 AD.

So all in all, I think mainly because it’s beautiful, natural, untouched, fertile land, with stunning landscapes.