Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* Proper nouns are specific names of people, places, or things (and are always capitalized).
"Sojourner" refers to a person who lives temporarily in a place. It's a general term that could apply to many different people.
For example:
* "The sojourner arrived in the village yesterday." (general term)
* "Sojourner Truth was a famous abolitionist." (proper noun, specific person)