Chassis flex throws off the geometry of the suspension. If the car is designed to have 3 degrees of negative camber when the wheel is turned at whatever angle and the flexing of the chassis allows the wheel to be at 4 degrees of negative camber then your not going to get maximum traction.
As far as I know, the Z32 chassis is pretty stiff for a street car. However as you move onto the track and the car starts seeing cornering forces that are much higher than anything a street car will encounter, even a relatively small amount of flex will become a bigger issue.
Some additional reading if you're a bit more math oriented:
