Why is the rate of photosynthesis low when light is too high or too low?

At low light, light is the limiting factor, so the plant cannot make enough energy and the rate is low. As light increases, the rate rises because more light energy is captured. But this only continues up to a point. Beyond an optimum intensity the rate levels off because some other factor, like carbon dioxide concentration or temperature, becomes limiting and light is no longer the bottleneck. At very high intensity the rate can actually fall because strong light can damage chlorophyll molecules, a process called photo-oxidation or photoinhibition, and intense light often comes with heat that can close stomata, reducing carbon dioxide intake. So both extremes are unfavourable, and plants photosynthesise best at moderate, optimum light.