Here's why:
* Proper nouns are names of specific people, places, or things. They are always capitalized. Examples include "John", "New York City", and "Amazon".
* Common nouns are general names for people, places, or things. They are not capitalized unless they begin a sentence. Examples include "man", "city", and "river".
"Hot dog" is a common noun that refers to a type of food. It's not the name of a specific brand or product.
However, if you're talking about a specific brand of hot dog, like "Oscar Mayer," then "Oscar Mayer" would be a proper noun.