Drive across the American South and the word barbecue keeps meaning something different — a different animal, a different cut, a different sauce or none. The boundaries are old and they follow supply.
The animal came from what the region raised
Regional styles formed around whichever livestock the local agricultural economy produced in quantity. Areas with extensive hog farming built their traditions on pork; cattle regions built theirs on beef.
That is why whole-hog and shoulder traditions cluster in one part of the country while brisket dominates another. The cooking method followed the carcass rather than the other way around.
Cuts that were cheap and tough are the ones that ended up in the pit, because long low cooking is precisely what makes connective tissue palatable.
Wood was a local material, not a choice
Smoke flavor comes from whatever hardwood grew nearby and was affordable in volume. Oak, hickory, pecan and various fruit woods each burn and smell differently.
Before wood could be shipped economically, a pitmaster used the local species by default. Generations of cooking with one wood set the regional expectation of what barbecue should taste like.
Today wood can be trucked anywhere, but the tradition it created has outlived the constraint that produced it.
Sauce began as preservation and seasoning practice
Vinegar based sauces reflect an era when acid served a preserving function as well as a flavoring one, and they suit fatty pork particularly well.
Tomato and molasses sweetened styles came later, following the availability of those ingredients and of commercial bottling. Mustard based sauces track particular settlement patterns.
In some traditions the meat is served barely sauced at all, on the reasoning that smoke and rendered fat are the point and sauce competes with them.
The sides are part of the regional definition
What arrives beside the meat is as regionally fixed as the meat itself, whether that is a particular slaw dressing, a specific bread, beans, or a starch that is unremarkable a few states away.
These accompaniments developed as the cheap, filling counterweight to an expensive protein at community gatherings, which is why they lean toward vinegar, starch and cabbage.
Why the styles have not merged
Barbecue is unusually place-attached because the equipment is immobile, the skill is learned in person, and customers hold strong expectations that punish deviation.
Restaurants that travel well have spread individual styles nationally, but the regional cores remain distinct because the local audience keeps them that way. Ordering the local style rather than the familiar one is the whole value of eating barbecue on a road trip.