Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns refer to specific, unique people, places, or things.
"Settlers" refers to any group of people who settle a new area. It's not specific to any particular group of people.
For example:
* Common noun: The settlers faced many hardships. (refers to any group of settlers)
* Proper noun: The Pilgrims were early settlers of Plymouth. (refers to a specific group of settlers)