As I understand it helicopter rotor blades keep a constant aerofoil section along their length, and are not twisted. Other rotating aerodynamic devices such as propellors, turbofan fans and wind turbines thicken and get a steeper angle towards the hub due to the lower incident airspeed (in the direction of rotation).
I'd guess it'd have something to do with variation of airspeed on the advancing and retreating sides maybe?
Edit: some are pointing out that this may in fact not be true, and that rotors can in fact have twist, just that it's less visually apparent. The chord at least would still appear to be constant
Edit 2: Since posting this I've taken a module in helicopter aerodynamics at uni, and the question is indeed wrong, blade twist is most definitely a feature of helicopter design.