One reason could be the effect of tail rotor thrust in a hover, or "translating tendency". The main rotor thrust has to be angled to compensate for this (either by rigging of the cyclic control, angle of the MR mast, or simply by manual cyclic input), which reduces the amount of available cyclic in that direction.
In forward flight, all single rotor helicopters have to compensate for disymmetry of lift due to the difference in airspeed of the retreating and advancing blade, which also affects the amount of available cyclic towards the retreating side. I'm not an expert, but I'd imagine that this could also have an effect on the allowable CG envelope.
The position of the fuel tank affects the CG, but not the CG _limitations_