Here's why:
* Short vowel sounds are typically found in closed syllables, which are syllables that end with a consonant. The word "must" has only one syllable, and it ends with a consonant ("t").
* Long vowel sounds are typically found in open syllables, which are syllables that end with a vowel.
Therefore, the vowel sound in "must" is short. It sounds like the vowel sound in words like "cut" or "sun".