As has been said, it's partly the more rural background. Farming and Ranching.... but it's not just that. Japanese people were often farmers for alot longer and for a period of Japanese history, Japan was one the most well armed countries on Earth....
It has to do with this. Southerners are a collection of four main groups. The descendants of English and French second and third class nobility and lower classes (from South England, and various parts of France), who quickly picked up firearms for defensive use because of the owning of slaves, because of the need to hunt, and because of the need to defend themselves from Indians.
Scots-Irish. Who, when this country was getting founded came from a war-torn, nasty place, prone to blood fueds, and a very serious sense of personal rights, personal honor, and personal ownership. I didn't really mention the Native Indians. Well, there weren't many, but much of their stuff rubbed onto the Scots-Irish (many Scots-Irish are also part Indian). At the same time, they also often fought the Natives, and those wars also seriously influenced making guns integral to their culture. In many ways, the best thing the Scots-Irish gave us all was the "get off my land!" instinct Americans of all ethnicities seem to have developed to some degree or another.
Germans. These were largely the builders of our best weapons early on. (interesting the name Sturm-Ruger as a US arms company). But, they also had a similar situation to the Scots-Irish. The Germany (and Germans) of today are a very different people than the ones who weren't that far (in memory) from the 30 Year's War, and still were having religious, social, and regional conflicts (they weren't even one country yet). Though they were more often settling in the North, the Southern settling ones tended to be very much like the Scots-Irish in mentality.
The last group are the Black Slaves. And their descendants, Black Americans. The only group not allowed to arm themselves. Modern blacks are either not very well armed, largely as a heritage of that (similar to many Eastern Europeans who were in a similar way when they get here). Or they are armed, as a counter to the past, and the oppression from that era (or just to protect themselves).
That's a short and haphazard explanation. (I could write a book on this subject).... but I think it kinda explains it.