As you climb higher the reducing thrust available causes the climb angle to decrease.
If you maintain a constant pitch angle as the climb angle decreases, your angle of attack will be increasing. This will cause IAS to decrease.
So to maintain a constant angle of attack, in order to maintain a constant IAS, you must reduce pitch angle to compensate for the reducing climb path angle.
WARNING TO PEDANTS....Yes I know that we should really be talking about constant EAS, but this is just another of those exam simplifications!