Here's why:
* Short vowel sounds are typically quick and sharp. The "a" in "brain" has a short, quick sound.
* Long vowel sounds are typically drawn out and often have the same sound as the letter's name (e.g., "a" in "ate").
While "brain" is spelled with an "ai," this combination doesn't always represent a long "a" sound. In this case, it represents the short vowel sound.