I would guess that because a majority of the time in a street car the wheels are going straight and require little attention. In an electric rack this requires little power because the alignment wants the car to go in a straight(ish) line, while in the hydraulic system is constantly dragging down the crank at a higher rate regardless of steering angle. I can see the electric rack being more efficient.
Later
When was the last time you *really* did something with your Z to call yourself a Z enthusiast?
