@EmilyGreen,
True to rumor: It gets hot.....REALLY HOT in the south during the summer.
And when it's hot, people gotta hydrate, right?
Well, my answer to your question would be that it is part economics, part taste, and part tradition.
If you didn't grow up in a hot region, what did your parents give you to drink in the summer?
Soda? That is rather expensive, and just consider what all that syrup will eventually do to a kid's kidneys after the copious amount necessary on a scorching August afternoon in northern MS.
Water? It gets bland after a while.
Kool Aid? (or similar) Yes, I was given gallons upon gallons of the stuff and as a kid, I loved it and had and still have my personal favorites. But.... as I grew up, Kool Aid was eventually edged out for the more satisfying, complex and superior taste of sweet tea over artificially sweetened sugar.
And let's face it: All sodas are basically glorified, artificially sweetened, carbonated Kool Aid.