Because light rips out electrons. Under the influence of light, the energy of electrons in atoms increases, and they can leave the plate. With a negative charge, the torn electrons are repelled from the plate and the electrometer is discharged. With a positive charge, the field returns the torn electrons back to the plate and the electrometer does not discharge.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.