Here's why:
* Adjectives describe nouns. "Ablaze" describes something that is on fire or burning brightly.
* Nouns are words that name people, places, things, or ideas.
For example:
* The house was ablaze. (Here, "ablaze" describes the house, which is a noun.)
While "ablaze" can sometimes be used as a predicate adjective following a linking verb (like "is" or "was"), it still functions as an adjective describing the subject of the sentence.