Original bbq sauce was made from meat juices and marinade from an actual barbecue. The stuff from the bottle is made to be similar to that.
Modern day BBQ sauce is actually made from ketchup and sweeteners like brown sugar. As to your original question, i guess it's called barbecue because it wants to be called that? I don't know, man! @Baloobas, Didn't know that. Im a stoner who loves knowledge and i love facts like that.