The more forward the C of G, the more aerodynamic effect is required from the horizontal stabilizer to maintain balanced flight (more nose up trim is required). Thus there is more down elevator available when you want to recover a spin at that C of G. At the aft C of G limit, the elevator is already trimmed down somewhat, so less range of elevator travel is available to apply nose down elevator control, and the airplane is balanced such that it is not so eager to lower the nose on its own (tail heavy), so more nose down elevator will be required.
In cruise flight, it is desirable to fly with a more aft C of G, as there will be less elevator deflection, and less drag - but the plane is also less spin tolerant.
While doing spin testing on a modified Cessna Caravan, I found the forward C of G spins were easily recovered, aft C of G spins were alarming. The plane recovered as required by regulation, but the spin was very flat, and full nose down elevator had to be held in for an extended period to get the nose below the horizon. If you spin behind the aft C of G limit, recovery from a spin is no longer assured for you, that really is the basis of the establishment of the aft limit.