Why specifically 16 hours of fasting?
There is nothing special about fasting exactly for 16 hours.
Incase you are finding it difficult, you can start by fasting for 14 hours and then increase it to 16 hours. The idea is to give a longer gap between your last meal of the previous day and the first meal of the next day so that your body is able to get more hours to heal itself.
If people were just told to fast between their last meal and the first meal the next day, they might not have fasted enough every day and given their body enough time to cure their disease.