I was always told it's because in the South you don't have problems with frost heaving. A slab up here would be cracked in two years, and the entire house would no longer be level and plumb after about five years. The ground gets waterlogged in late fall and then freezes, and the frost will pop just about anything back up out of the ground. Some people get shrubberies and trees on sale in fall, plant them, and then get frustrated because they pop right back out in spring and die. So basements here go below the frost line (>4 feet deep). Another reason, we need mechanical space for a serious furnace, and you can't keep the oil tank outside or your lines will freeze. My oil-burner furnace + tank + repair kit + replacement filters take up a space the size of a small bedroom.