Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They describe how, when, where, or to what extent something is done.
* Adjectives describe nouns or pronouns.
In a sentence, "boldly" would tell us *how* something is done. For example:
* "He boldly walked into the lion's den." (modifies the verb "walked")
* "She had a boldly painted mural on her wall." (modifies the adjective "painted")
The adjective form of "boldly" is "bold."