Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized. Examples: John, Paris, Mount Everest.
* Common nouns are general names of people, places, or things. They are not capitalized unless they are at the beginning of a sentence. Examples: man, city, mountain.
"Priest" is a common noun because it refers to a general title for a religious leader.
However, if you are referring to a specific priest with a name, then that name would be a proper noun and capitalized. For example: "Father John" or "Reverend Smith" would be proper nouns.