Here's why:
* The "oo" sound is short: It's pronounced like the "oo" in "look" or "book," not like the long "oo" in "moon" or "food."
* Short vowels are typically followed by one consonant: The "oo" in "igloo" is followed by the single consonant "l."
Therefore, the "oo" in "igloo" is a short vowel sound.