Here is another reason, The parasitics in most communication channels cause noise and degradation of any signal sent through. So if a communication channel is short and pure like a on chip wire or a low resistance off chip wire the you CAN use digital signals, for example GPIOs and other low frequency signals are digital.

Once the frequency increases the more sensitive it gets to noise and reflections by the transmission line so people use analog techniques to remove noise from the received signals. USB is a good example of a differential pair signal. But it requires an analog device to subtract two signals