Here's why:
* Adjectives describe nouns. They tell us more about the noun.
* Common nouns are general names for people, places, or things (e.g., dog, city, chair).
Examples:
* Bright sun (bright describes the noun "sun")
* The bright light (bright describes the noun "light")
To make "bright" a noun, you could use it as an abstract noun:
* The brightness of the sun was blinding.