It's a Southern phenomenon. One theory is that it's something the borderland people (northern English & Scotts-Irish) brought with them when they settled the South.

Here's an excerpt from "Albion Seed" on this theory:

In the backcountry, before the end of the eighteenth century, a familiar form of evangelical religion was the camp meeting. This was an outdoor gathering, commonly convened in some sylvan setting, where a large number of people worshiped together for several days. Many historians have mistakenly believed that the camp meeting was invented on the American frontier. In fact it was transplanted to America from the border counties of Britain, where it was well established by the eighteenth century. Even the Anglican population of that region often met in outdoor "field meetings" during the eighteenth century. So also did Scottish Presbyterians who held frequent "Holy Fairs," which were camp meetings by another name.