Here's why:
* It describes a noun: "The movie was awful." (Awful describes the movie)
* It can be modified by adverbs: "The food was incredibly awful." (Incredibly modifies awful)
However, "awful" can also be used as an adverb in informal contexts:
* "That joke was awful funny." (Awful modifies the adjective "funny")
It's important to note that using "awful" as an adverb is considered less formal and is more common in spoken language.