I think it has everything to do with region and general availability of a specific drink over time. For instance Coke originated in Georgia so it makes sense a lot of people from that area call everything a 'Coke', and Coke was the first general soda widely available worldwide. Soda and pop or the combination was just terms that stuck around since the 1800s with just saying pop being slang for soda pop.
So let me just get this straight. If I one day travel to Georgia and go into a bar and ask for a coke, they're going to turn around and ask what soda I want? My mom and sis already had stories of a surprising amount of translation problems when they visited NYC.