Here's why:
* Common nouns refer to general things or people (e.g., cat, dog, city, drink). They are not capitalized unless they start a sentence.
* Proper nouns are specific names of people, places, or things (e.g., John, Paris, Coca-Cola). They are always capitalized.
"Fizzy fizz" is a descriptive phrase for a type of drink that is bubbly and fizzy. It doesn't refer to a specific brand or product.