Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* "Community" doesn't refer to a specific group of people or a particular place, but to the general concept of a group of people living in the same place or sharing common interests.
Examples:
* The community came together to help after the storm. (general concept of a group of people)
* The community of artists in the city is thriving. (general concept of a group with shared interests)
Contrast this with a proper noun:
* "The Greenwood Community is holding a fundraiser." (specific name of a community)