Well the first and most obvious reason is good ole Southern hospitality; even a Yankee will admit that you just can't beat it. As a missionary you need to not only be hospitable but you need to be able to receive hospitality; this is absolutely essential especially when being received by other nations.
There is a level of manners and politeness that you can only find in a home-grown Southerner. We all say our "yes ma'am"s and "no ma'am"s, our "yes sir"s and "no sir"s. It has been deeply engrained in every Southerner by the belt, the soap, the switch, and the smack our mamas and daddys gave us when we forgot our manners.
Another part of this applies to strangers and people we can't stand. If you go down to the South you'll find you have people you've never met nodding a hello your way, people in the cars passing by will wave, and every person you pass on the street will ask you how it's going. This is something that you can't teach, it's just apart of our Southern DNA. When we Southern missionaries walk into a new place, you'd think we'd been there before cause of how friendly we are. Missionaries are also prone to live with groups of people but Southerners are the best community dwellers you'd ever meet, we may hate your guts but we ain't gonna let that stop us from being polite.
You can't scare us, no matter what food you put in front of us. Sure it won't be as good as mama's cooking but if our stomachs, arteries and bodies have survived all that grease, all these years then it doesn't matter what kind of scary ethnic food you put in front of us...we'll survive. We'll wash it down with a glass of ghetto sweet tea we had to make with these Red Coat's tea bags and sugar cubes.
Because the South is so warm and because once upon a time they won one little war (supposedly that gives them territorial rights), we are accustomed to dealing with Yankee foreigners. So when it comes to living with other cultures and having to cope, shoot we ain't got no problem doing that...been dealing with foreigners for years.
Now I could go into our hard, farm-driven work ethic, our thick family values, and so on; but y'all already know that you can't go wrong with the South.