Lower digitisation rate for long distance communcation
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!