Here's why:
* Sound: The "y" in "fairy" makes the same sound as the vowel "i" in "fish".
* Syllable: The "y" forms the second syllable of the word "fairy" (fair-y). Vowels are necessary to create syllables.
While "y" can sometimes act as a consonant (like in "yes" or "yellow"), in "fairy" it functions as a vowel.