Here's why:
* Comparative and superlative forms express degrees of comparison. For example, "big" (base form), "bigger" (comparative), "biggest" (superlative).
* Adverbs express how, when, where, or to what extent something is done. They don't usually express degrees in the same way adjectives do.
Instead of using comparative and superlative forms, you might use other adverbs or phrases to express varying degrees of fearfulness:
* More fearfully (instead of "fearfuller")
* Most fearfully (instead of "fearfullest")
* With great fear
* With increasing fear
* Terrified
* Panicked
Example:
* Instead of: "He ran fearfuller than before."
* You could say: "He ran more fearfully than before." or "He ran with increasing fear."