Dr. Drang explained it best a year or two ago. Basically if we stuck with sun time, then sunrise in Chicago (and other places with similar latitude) would be 4:

Dr. Drang explained it best a year or two ago. Basically if we stuck with sun time, then sunrise in Chicago (and other places with similar latitude) would be 4:30 AM in the summer which would be kind of a waste unless everyone just woke up an hour earlier anyway. If we didn't change back for winter, then the sun would set before most people got off work during winter which would be very hard as office workers need some sunlight during the day.