Lower digitisation rate for long distance communcation Why is it that usually lower digitisation rates are used for long distance communications? e.g. sound is highly distorted in undersea communications, is this because of the lower digitisation rate used?

why lower digitisation rate is preferable?
At long distances, your transmit signal loses a lot of power. As a result of that, the SNR at the receiver is possibly relatively low.

A low SNR means that you cannot transport many bits per second over that channel (Shannon limit).

Not having many bit/s means you can't push much data across, which means you must reduce the account of data in the digitized speech. It's that simple!