Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized. Examples: John, London, Mars, Amazon River.
* Common nouns are general names for people, places, or things. They are not capitalized unless they begin a sentence. Examples: man, city, planet, river.
"Scientist" is a general term for someone who studies science. It's a common noun.
You would only capitalize "Scientist" if it was the beginning of a sentence or part of a title.