The name "pop" from soda pop derives from the the original bottles that were first used. The first glass bottles used had a marble within them. Once filled the pressure from the carbonation within the bottle would hold the marble up in the neck of the bottle thus creating an airtight seal. To open the bottle, you would press down on the marble with a finger to release the seal. When the seal was broken there was an audible popping sound and thus you get the name "pop".
It used to be called soda pop in referring to any carbonated soft drink. It got shortened to soda OR pop decades ago. Around here if you stick ice cream in your soda pop you have an ice cream float, or as it again also got shortened to "float".
The US is a big country, and there are different dialects in different regions, and it's not just a difference between soda and pop (or cola, or even "Coke" being referred to in the general sense for a cola) or soda pop. Why ask just about carbonated beverages? You could ask the same thing about why pancakes have a different term around the country....or why Minnesota plays "duck, duck, gray duck" when the rest of the country plays "duck, duck, goose." Is it a couch, a sofa, a davenport, or a chesterfield? Some dialects are region wide, some are state wide, and some dialects are only a small segment of a state's population. I think PBS did a show a couple years back about the different dialects in the US.
Soda is called pop because they are more country and do things different