Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* Proper nouns are specific names for individual people, places, or things.
"Savannahs" refers to a general type of grassland ecosystem, not a specific place.
Example:
* Common noun: The savannahs of Africa are home to many animals.
* Proper noun: The Serengeti is a famous savannah in Tanzania.