In fact, it doesn't. We find that the photon is most likely to strike the wall at a point within one of the bright bands. In the center of one of the dark bands, the probability of the photon striking is zero. The pattern is the same whether the light is bright or dim. We would see a similar histogram whether one trillion photons were emitted all in one hour, or whether one photon were emitted per hour, for a trillion hours.
Physicists say that the interference pattern manifests itself as a probability density function.
That means that the interference pattern predicts where the photon is likely to strike. In the areas where the bands were the most bright, the photon will be most likely to strike. In the areas where the bands were dark, the photon will be less likely to strike. This simple experiment encapsulates the deep mystery of quantum mechanics. Physicists and philosophers have struggled mightily to explain this phenomenon ever since it was discovered.