This is because photoelectric effect is a phenomenon in which electrically charged particles are emitted from a material when light is incident on it.
The light generates the energy which causes the material to emit electrons.
These electrons are only emitted at certain values of light energy, this thus shows that the light has to be quantized since electrons are not emitted over a range of values of light energy.
Thus, to fully describe the photoelectric effect, only light needs to be quantized.
