The alternating current across the capacitor plates will increase with increasing frequency. As more frequently the voltage changes, more fast the current will flow. Hence reducing the resistance offered by the capacitor that is Capacitive Reactance.
Also by formulation Capacitive reactance is given as

Where, X_C is the Capacitive reactance, f is the frequency of the supply and C is the Capacitance.
We see that Capacitive reactance and Frequency has inverse relationship.
Hence, the reactance provided by a capacitor to an alternating current decreases with increasing frequency.