Impedance, Capacitance and Resistance

Impedance

When an incorrect impedance is chosen, the impedance mismatch causes reflections of "standing waves" on the cable.  These cancel out portions of other waves, or can re-reflect and produce false data on the line.

Capacitance
Important in analog, capacitance is doubly critical at digital frequencies.  This is not only because of the relatively high frequency involved but also because of the nature of the waveform.  Data is essentially a square wave, with the most critical point being the transition between zero and one, and between one and zero.  This transition is, by definition, instantaneous.  It is therefore, an extremely rapid state change, which is equivalent to an extremely high frequency.  Anything which "rounds off", limits, changes or obscures that transition point will add to the bit errors, jitter and cable length limitations on that system.

Resistance
Resistance is the other significant factor in analog audio.  However, it is often easier to reduce the capacitance in a cable design than reduce the resistance.  Reducing the resistance means putting in a wire of larger gage.  However, to keep such a cable design at the same capacitance as before (not even reducing it) means adding much more plastic.  Therefore, such a cable design will be a lot bigger, and a lot more expensive.  Most audio cable, analog or digital, are centered around two gage sizes, 22AWG and 24 AWG.