Here's why:
* Common nouns refer to general categories of people, places, or things. They are not specific to a particular person, place, or thing.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Since "journalist" refers to any person who works as a journalist, it is a common noun. If you were referring to a specific journalist, like "Bob Woodward," then that would be a proper noun.