Here's why:
* Common nouns refer to general things or places. They are not specific to a particular individual, place, or thing.
* Proper nouns are specific names for people, places, or things. They are always capitalized.
For example:
* Common noun: meadow
* Proper noun: Central Park (a specific park)
You could say "The meadow was filled with wildflowers" without referring to a specific meadow.