Because fasting can be pretty miserable.
Yom Kippur is about atonement, cleansing your soul and starting anew. And since atonement doesn’t come easy, it also requires self-affliction, mostly performed through fasting. But if the fasting and the no washing and the no wearing leather shoes and the ban on sexual relations isn’t enough, next year you might try to inserting a caffeine suppository into your anus.