
Diffraction is a phenomenon that is easily explained by wave interference. Light of course is a wave so why wouldn't this be the case?
But what happens if you send only one photon at a time? Surely then there's no interference.
Like it or not, even when sending one photon at a time you discover strange phenomenon that do not traditionally make sense.
They only make sense if you truly accept how bizarre our world is.
Source: QED: The Strange Theory of Light and Matter