Well from what I have heard people called soda "pop "probally because of the sound it made when opened.Also soda pop is another terminolagy word that fits the sound.Now people call soda by the flavor which it represents.
Just depends where you are in the country. I'm told that in some parts, it's all "Coke" as in orange Coke, root beer Coke, etc. Sort of the same as pancakes, flapjacks, johnny cakes, etc. for the same breakfast food.