Here's why:
* Common nouns refer to general types of people, places, or things.
* Proper nouns are specific names of people, places, or things and are capitalized.
"Anaconda" refers to a type of large snake, not a specific individual snake. It's a general term that can apply to any snake of that species.
Here's an example:
* Common noun: The anaconda is a powerful predator.
* Proper noun: The Amazon River is home to many anacondas.
In this example, "anaconda" is used to refer to the species in general, while "Amazon River" is a specific name and capitalized.