Final answer:
Elie didn't fast on Yom Kippur because he was too ill to do so, as described in Elie Wiesel's memoir, Night.
Step-by-step explanation:
The reason Elie didn't fast on Yom Kippur was because he was too ill to fast. In Elie Wiesel's memoir, Night, he describes his experiences during the Holocaust, including his time in concentration camps. During his time in the camps, his physical health deteriorated, making it impossible for him to observe the religious practice of fasting on Yom Kippur. This was a result of the harsh conditions and extreme malnutrition he faced in the camps.