Here's why:
* Common nouns refer to general categories of people, places, things, or ideas. They are not capitalized unless they start a sentence.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
Examples:
* Common noun: cartoon
* Proper noun: The Simpsons (a specific cartoon)
Since "cartoon" refers to a general category of animated drawings or films, it's a common noun.