Here's why:
* Contrast involves showing differences between two things. The author is using the phrase "not for" to highlight what the reader *is* looking for, by pointing out what it *isn't*.
* Antonyms are words with opposite meanings. While the author may be using words that are antonyms to what the reader wants, the overall effect is to highlight the contrast and guide the reader towards the desired meaning.
For example, imagine an author writing about a character who is not adventurous, but prefers a quiet life. By describing the character as "not for exploring new places," the author highlights the character's preference for routine and stability, guiding the reader to understand their personality.