It's the combination of all the regional styles and flavors. In the south and southeast, they do primarily pork. In Texas, they do primarily beef brisket. In Kansas City, we cook it all. Barbeque is frequently smoked, though what to smoke it with is another bone of contention.
Most people use hickory, but mesquite is growing in popularity.
Barbequers in West Texas have been smoking with this spiny shrub for years. Hell, if we had hickory in Texas, we'd use that instead. Try making this barbeque sauce for Dear Old Dad this Father's Day. It's Not Just for Breakfast Anymore. Simmer just until heated through, stirring occasionally.