Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* Proper nouns name specific people, places, or things, and they are always capitalized.
"Discovery" is a general term that can refer to any act of finding or learning something new. It's not the name of a specific person, place, or thing.
Here are some examples:
* Common noun: The discovery of penicillin was a major medical breakthrough.
* Proper noun: Columbus made a discovery that changed the course of history.