Here's why:
* Common nouns refer to general, non-specific things. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things, and they are always capitalized.
"Empire" is a general term for a large political unit ruled by a single sovereign or government. It doesn't refer to a specific empire.
Examples:
* Common noun: "The empire was vast and powerful."
* Proper noun: "The Roman Empire was a major force in ancient history."