Why is reducing caloric intake insufficient for a person to loose weight in a healthy way?

Because there are several factors that determine weight loss or gain and reducing calorific intake is just one of them (for reducing weight loss). Other factors that should be considered is exercise. Doing regular exercise ensures that the metabolic rate of the body is increased so much of the calories are burned even after intake. Having a lower metabolic rate can result to increased weight gain even with low calorie intake.