Final Answer:
It is because of increases the electric field strength within the capacitor, resulting in reduced charge storage.
Step-by-step explanation:
The capacitance of a capacitor is inversely proportional to the electric field between its plates.
When a dielectric is introduced, it polarizes in response to the electric field, creating an opposing field that weakens the overall electric field between the plates.
This, in turn, reduces the capacitance.
Dielectric materials have the ability to support electric dipoles, causing an internal polarization.
As these dipoles align with the electric field, they counteract a portion of the original field, effectively reducing the net electric field experienced by the capacitor.
Since capacitance is directly related to the electric field strength, the introduction of a dielectric decreases the capacitance.
In practical terms, the dielectric material enhances the capacitor's ability to store charge by allowing a higher voltage across the plates without an increase in charge.
This property is exploited in various applications such as increasing the energy storage capacity of capacitors in electronic circuits.
To sum up, the decrease in capacitance when a dielectric material is added is attributed to the reduced electric field strength between the capacitor plates due to the polarization of the dielectric material.