Here's why:
* Short vowel sounds are typically quick and sharp, like the sound of "e" in "bed" or "a" in "cat".
* Long vowel sounds are typically longer and smoother, often sounding like the letter name, like the sound of "a" in "ate" or "e" in "me".
The "ea" in "bread" does not sound like the letter name "e", so it's a short vowel sound, similar to the "e" in "bed".