Here's why:
* Proper nouns name specific people, places, or things. They are capitalized. Examples: John, London, Eiffel Tower.
* "Puerto Rican" describes someone or something from Puerto Rico. It is an adjective and is not capitalized unless it is part of a proper noun, like "Puerto Rican cuisine".
Instead, "Puerto Rican" is an adjective. It's similar to saying "American" or "Canadian".
Let me know if you have any other questions!