Here's why:
* Spelling: The "e" in "reason" is followed by a consonant, which typically indicates a short vowel sound.
* Pronunciation: The "e" sound in "reason" is a distinct, short sound, not drawn out like a long "e" sound.
If you want to hear the difference, try saying the words "reason" and "reasoned". You'll notice that the "e" sound is shorter in "reason" and longer in "reasoned."