At high frequencies the skin depth of copper decreases and the signal is confined to a small amount of material, which provides a high ohmic resistance to the s

At high frequencies the skin depth of copper decreases and the signal is confined to a small amount of material, which provides a high ohmic resistance to the signal. So for copper there is always a maximum frequency for the signal thus limiting the bandwidth. For air the signal propagates as a free EM wave, essentially in the vacuum, and isn't band limited by the material its traveling through. This is my understanding at least, I am open to correction/ nuance.