It has everything to do with superposition. A photon can only exhibit wave-like effects (i.e. interference) as long as its position ('x') state exists in a supe

It has everything to do with superposition. A photon can only exhibit wave-like effects (i.e. interference) as long as its position ('x') state exists in a superposition. Measuring which slit the particle travels through collapses the particle's position into a pure state, thereby removing the interference effect.