Because...... it is extremely important for your bones and arteries to get adequate fat soluble vitamins to assure that calcium goes into bone and NOT your arteries. (nutritionfacts.org)
Long story short: you need to eat enough protein, but without adequate fats in your ketogenic diet, you will eventually run out of energy. (womenshealthandfitness.com.au)
I also look for adequate fat intake when trying to find underlying causes of constipation. (drlaurell.com)