Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They describe how, when, where, or to what extent an action is performed.
* Nouns are words that name people, places, things, or ideas.
In a sentence, "fluently" would describe the manner in which something is done. For example:
* She speaks French fluently. ("Fluently" modifies the verb "speaks" and tells us how she speaks French.)