Here's why:
* Common nouns refer to general categories of people, places, things, or ideas.
* Proper nouns are specific names of people, places, or things, and are capitalized.
Examples:
* Common noun: legalism (the practice of emphasizing strict adherence to the letter of the law)
* Proper noun: John (a specific person), London (a specific city), The Bible (a specific book)
Since "legalism" is a general term for a concept, it is a common noun.