Here's why:
* It modifies a verb, adjective, or another adverb: It tells us *how much* or *to what extent* something is happening. For example:
* "He mostly likes pizza." (modifies the verb "likes")
* "I am mostly happy." (modifies the adjective "happy")
* "She walked mostly silently." (modifies the adverb "silently")
* It can also be used as a pronoun: This is less common, but it's possible:
* "Mostly are happy with the results." (Here "mostly" acts as a pronoun referring to the majority of people)
While it can be used as a pronoun in some cases, its most common use is as an adverb.