Here's why:
* Common nouns refer to general things or people. They are not capitalized.
* Proper nouns refer to specific people, places, or things. They are capitalized.
Examples:
* Common noun: processor (a general term for a device that processes information)
* Proper noun: Intel Core i9 (a specific type of processor made by Intel)
Since "processor" is a general term for any device that processes information, it is a common noun.