You eliminate weight and complexity by using air cooling. No water pumps, radiators, expansion tanks and ~6-10 L of coolant. The water pumps use power from

You eliminate weight and complexity by using air cooling. No water pumps, radiators, expansion tanks and ~6-10 L of coolant. The water pumps use power from the engine, the casting of the water jackets results in a heavier engine, and the radiator and coolant all add weight. In the case of a rear engined car, eliminating water cooling helps even more because of the need to transport the coolant all the way to the front and back again. Mind you, Porsche eventually accepted this compromise, and to be fair, they were already doing it for oil lines.