Long runs of copper make great anntenae for picking up spurious EMI. By using an extremely high frequency for the carrier (IR or visible light) it makes it very easy to shield the "conductor" of the signal with any material that is opaque to light.
That means the only source for EMI is in the electronics within the amp/source and the speaker, which is much easier to shield or otherwise mitigate than a long cable.
Bigger cables mean they can carry more current for the signal, and then the EMI becomes a much smaller noise floor as a percentage of the larger signal current.
Yeah, I was going to mention digital, like using a USB mic instead of plugging an analog mic into the old phono jack in the PC.
If the comm protocol can detect errors then it will not pick up any noise in the signal itself. But the copper can still carry EMI to the other electronics other than by being carried by the signal.
@Pat
All true, but I find most speaker RFI in a computer setup is near-field not far-field. Meaning even relatively short runs, since it is adjacent to power carrying lines, pick up a **lot** of RFI. So even short runs of optical line really help. Also the fact that it is digital helps far more than it being optical, but optical is just nice anyway :)
@icedquinn