Here's why:
* Short vowels have a short, quick sound. They are typically found in one-syllable words or in the unstressed syllable of a multi-syllable word.
* In "man", the vowel sound is short and quick. It's not drawn out like a long vowel.
If you were to say "man", you wouldn't pronounce the "a" like the "a" in "father". Instead, you would pronounce it with a short, quick sound.