Photoelectric effect is only observed when electrons reach the positive plate and to reach the positive plate they must have sufficient energy. A part of this required energy is the electrons kinetic energy and the rest comes from the energy of the incident light. You must agree that all the electrons don't have the same energy so all the electrons do not require the same amount of energy for emission from the metal plate. Thus, light of lower frequencies may not provide the energy required by some of the electrons for emission. You will easily understand this with an example. Suppose that there are 50 electrons in a certain area of the metal. 10 of them require only 6 eV of energy for emission, other 10 of them require 10 eV energy, and the rest of them require 20 eV energy. 395
So, Light having energy,say 25eV(of greater frequency) will release more electrons than light of 15 eV energy ( of lower frequency)
Edit 1: Consider 2 beams of light having same intensity (i.e.,same energy) but beam 1 has higher frequency, so each photon of it have higher frequency and thus higher energy too( from planks law, E=hv). So, a lesser number of photons of beam 1 have the same energy as beam 2( having greater number of photons). Thus, beam 1 and beam 2 having same intensities have different number of photons.(You also must know that all the photons do not successfully emit an electron, this happens only for saturation current) See what happens at saturation current: the photons of beam 1 impart greater kinetic energy to the electrons, so the electrons move faster as compared to the electrons emitted by beam 2, which impart lower kinetic energy. But, there are lesser number of photons in beam 1. So the net effect is that the saturation current due to beam 1 have lesser number of photoelectrons but those photoelectrons move faster! But the photoelectrons due to beam 2 are greater but move slowly. As a result the rate of flow of " charge" remains same in both cases. Since current is defined as " rate " of flow of " charge" so current due to both the beams have the same value! 395