Here's why:
* Short vowel words typically have a single vowel sound followed by one or two consonants. Examples are "cat," "dog," "hop," and "run."
* "Until" has two vowel sounds:
* The "i" in "until" makes the long "i" sound.
* The "u" makes a schwa sound (like the "a" in "about").
Therefore, "until" is a two-syllable word with long vowel sounds, not a short vowel word.