Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* "Incessantly" describes the manner in which something happens, meaning "without stopping" or "continuously."
For example:
* "The rain fell incessantly for days." (modifies the verb "fell")
* "He was incessantly talking about his new car." (modifies the verb "talking")