It all comes down to the exactness of the transfer down the cable. In an optical transmission, the light does not travel straight down the middle of the cable it constantly bounces back and forth down the "sides". The better the reflective qualities of the interior of the cable, the less diffusion of the light from emitter to receiver. Blurring (diffusion) of the light can cause not just loss of data but mistaken data (and the more often it happens, the less likely the error correcting algorithms can correct it accurately). So, in short, the better the medium, the more exact a replication of the data through transmission.