Here's why:
* Proper nouns are names of specific people, places, or things (e.g., John, London, the Eiffel Tower).
* Common nouns are general names for people, places, or things (e.g., man, city, tower).
"Iceberg" is a common noun because it refers to any large mass of ice floating in the sea.
However, if you're talking about a specific iceberg, you might give it a name. For example, the famous iceberg that sank the Titanic could be called "The Titanic Iceberg," and in this case, it would become a proper noun.