Here is a pretty complete history : http://www.amazingribs.com/BBQ_articles/barbecue_history.html

"BBQ" was, and is a good way to make cheap, tough pieces (cuts of meat other people did not want) of meat edible. The slow cooking breaks down the connective tissues found in these gristly pieces and makes them more chewable. Traditionally, BBQ was primarily a style for poor, rural people. Who, in the US, are numerous in the Southern states.