Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us *how*, *when*, *where*, or *to what extent* something happens.
* "Suddenly" describes the manner in which something happens (it happens quickly and unexpectedly).
For example:
* The lights suddenly went out. (modifies the verb "went out")
* He was suddenly very tired. (modifies the adjective "tired")
* She stopped suddenly. (modifies the verb "stopped")