Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are part of a proper noun.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common Noun: a government
* Proper Noun: the United States Government
* Common Noun: a country
* Proper Noun: France
* Common Noun: a system of rule
* Proper Noun: Democracy (when referring to a specific system like "American Democracy")
Therefore, while "Democracy" can be used as a proper noun when referring to a specific system, it is generally a common noun.