Why do elderly people often get hypernatremia?

Water depletion. Great question, one that many in the med field don't quite understand. Hypernatremia (high blood sodium level) is a water dehydration state. Hyponatremia is a water excess ('intoxicated' state--it takes a mental disorder to drink so much water that it lowers serum sodium). When elderly don't drink enough water, are on a diuretic, gets a fever or uti, they easily get water depleted, hypernatremic.
Poor water drinking. Elderly have impaired thirst & also very often limit their intake of fluids to try to limit incontinence.