Noise in digital system exists only in the transition spikes (ie) places where there are 1 to 0 transition and 0 to 1 transition. This noise can easily be eliminated using all pass repeater filters.

But in analog communication system the noise is throughout the signal and so we can't use an all pass repeater to remove noise. We can't clearly say which is noise and which is signal in an analog signal. But we can do it easily in a digital signal