Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* Proper nouns are specific names of people, places, or things.
Since "department" can refer to any division within a company, organization, or government, it's a general term and thus a common noun.
Examples:
* Common Noun: The department is busy today.
* Proper Noun: The Marketing Department is having a meeting.