Here's why:
* Preposition: "Across" shows the relationship between a noun or pronoun and another part of the sentence, often indicating direction or position.
* Example: "The ball rolled across the field."
* Adjective: An adjective describes or modifies a noun.
* Example: "The red ball rolled across the field." ("Red" is the adjective describing the ball)
While "across" can sometimes be used as an adverb, describing how something happens, it's less common than its use as a preposition.
* Example: "The ball moved across the field quickly." ("Across" describes the movement of the ball)
In summary, "across" is mainly a preposition, and though occasionally used as an adverb, it's rarely considered an adjective.