Quite possibly because if all your life you have relied on dietary carbs as a source of energy, you do not switch readily to burning mainly ketones and fatty acids. It can take quite a long time to become fully "keto-adapted". Think marathon runners who "hit the wall" about 20 miles in to a run. They have run out of stored glucose (glycogen) and because they cannot switch immediately to fat, need to refuel on glucose to regain energy.