My suggestion would be the increased disc loading in modern helicopters.
More powerful engines allow designers to have smaller diameter rotors that just have to work harder to shift enough air downwards - rotor rpms are higher than older helos and so Nr will tend to decay quicker due to the extra drag.
Add in cambered aerofoil sections - high lift usually incurs drag penalties - and the very responsive nature of semi-rigid rotor systems and you have a reasonable recipe for lively Nr characteristics.