Richard,
The main reason is that a rotor blade spends much of its time flying sideways, when compared to a prop. Rotor design engineers study rotor performance in the flight regimes that ShyTorque has already mentioned (in that case it was fatigue life evaluation). Each design point (eg speed) has an optimum design, with hover looking very much like the prop you mention. Cruise requires a combination of good tip stall performance for the retreating side and good flutter stability in general. The latest machines now also have anhedral tips.
This is a complicated area, so i would start by reading Prouty "Helicopter Aerodynamics" from HeloBooks. Delta 3, has done some rotor simulation work, so is also a knowledgable chap.