I'm not too sure if this is totally right, but I'm sure an airframe man will correct me if I'm wrong.
I know most modern large helicopters have symmetrical section main blades. One of the properties of this type of blade, is that they do not produce any lift when they are in flat pitch, so when the helicopter is running on the ground with the collective fully down, there is no lift from the blades.
If a blade with an aerofoil section was used, lift would always be produced whenever the blades are rotating, so the only way to prevent lift whilst on the ground would be to put the blades into negative pitch. ( which causes vibration and stress on the airframe).
Mr G