Here's why:
* Common nouns refer to general things, not specific people, places, or things. They are not capitalized.
* Proper nouns refer to specific people, places, or things, and they are capitalized.
"Tradition" refers to a general concept, not a specific one.
For example, "The family has a tradition of gathering for Thanksgiving." This is a general idea of a tradition.
However, "The Thanksgiving tradition" would be considered a proper noun because it refers to a specific tradition with a name.