The major reasons are as follows:The electrostatic field inside a conductor is zero.The electrostatic field is normal to the surface at points on the surface of a charged conductor.Due to these two reasons, no or very negligible amount of work is done in moving a test charge on the surface and within the conductor. In simpler terms, the potential difference between two points on the surface and within the conductor is zero. Hence, the electrostatic potential is constant throughout the volume of the conductor and the value is the same as that at the surface.