Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things and are always capitalized.
"South Indians" refers to a general group of people from the southern part of India. It's not a specific name.
Examples:
* Common noun: Dog (refers to any dog)
* Proper noun: Buddy (refers to a specific dog)
* Common noun: City (refers to any city)
* Proper noun: Mumbai (refers to a specific city)