Here's why:
* Short vowel sounds are typically quick and sharp, and often found in closed syllables (syllables that end in a consonant). The "ai" in "snail" is a closed syllable, and the sound is similar to the "a" in "cat."
* Long vowel sounds are usually drawn out, and often found in open syllables (syllables that end in a vowel).
So, while "snail" has two vowels, the sound is a short vowel sound.