Possibly to save energy. Possibly to increase performance. Power steering takes juice. It also adds weight, which takes more juice to move, and could reduce the performance. Do many other roadsters have power steering?
With a car that small and light, power steering isn't really necessary. It adds weight and diminishes the driver's ability to feel what the car is doing and feel the road through the wheel.
Exactly. At that point it's more of a trade-off between luxury and performance.