How about this?

With more forward CG, for the same flight weight and speed, the stick is further back, causing it to be closer to your crotch, so you can deflect the stick less to the sides, thus adversely affecting roll rate.

Don't like that? Maybe this.

With more forward CG, for the same flight speed, the stick is further back. Creates more downforce by the tail, which means the wing has to make more lift and operate at higher angle of attack for the same flight speed (total upforce require by the wing is increased, even though weight is the same, and steady, 1 g gliding flight because the tail is pushing down harder if the CG is more forward). At the higher angle of attack, there is a shift in the differential lift created by deflecting the ailerons because the wing itself is operating at a higher angle of attack. Adverse yaw is increased, and overall roll rate diminishes slightly.

Unloading the wing by pushing the stick forward greatly reduces adverse yaw, so the net result is you can get a higher roll rate with a sailplane if the wing is unloaded and you are flying slow. At least, that last part has been my experience with my Nimbus 3.

Steve Leonard