Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are the first word in a sentence or part of a proper noun.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
For example, "Superman" is a proper noun because it refers to a specific comic strip character. However, "comic strip" is a general term for a sequence of drawings that tells a story, and therefore it is a common noun.