|laziness, impatience, and hubris|
The terms 'soda' and 'pop' are both shortened forms of 'soda-pop', a moniker given to this type of beverage by a confused person who believed the popping bubbles were due to the presense of soda (sodium bicarbonate), as with certain popular bathtub toys. This is of course wrong, as if you put enough soda in the beverage to make it fizzle and pop, you wouldn't be able to stand to drink it. Soda tastes pretty nasty. Since the popping actually comes from carbonation, the correct term would have been 'carbo-pop', but for some reason that never caught on. (Go figure.)
What people call such beverages is highly regional. Most of the midwestern US calls them 'pop', but 'soda' is more prevalent in the south. Other regions have their own preferences. I believe the term 'coke' is used mostly in areas where Coca-Cola has a much stronger influence than Pepsico -- near Atlanta, and in southern California, for example, and in many countries outside the US. (Coke is more international than Pepsi.) 'fizzy' is I believe used almost exclusively outside the US. I've never heard it called 'tonic', so I imagine that comes from outside the US as well. (To me, tonic is anything zealously marketed as the solution to all problems, and 'fizzy' would be champaign, which I've only actually ever seen on television.)
Due to the highly regional nature of these terms, the advertising industry in the US has adopted the neutral term 'soft drink' for almost all national advertising; hence, people who call it a 'soft drink' probably watch too much television in lieu of interacting with real people. The trouble is, the term 'soft drink' can also be used of non-carbonated beverages, including Hi-C and Kool-Aid.
The term 'carbonated beverage' is used primarily by people who are aware of these issues.
 Which is actually a bit east of the middle, but that etymology is another discussion.
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