The reason for reciprocity failure is instability of latent subimage. A silver halide crystal needs to be hit by several photons before a stable latent image is formed. If the illuminance is very low, photons hit a crystal very infrequently and thus latent subimage can be destroyed and go back to square one before additional photons strike the same crystal.
The lifetime of the latent subimage, as well as the “threshold” of the latent image depend a lot on the emulsion crystal technology as well as the chemical sensitization techniques used. Generally speaking, small tabular grain crystals are the best.