Why are officials in the Song Dynasty often demoted, and they are all demoted to Hainan, not other places?

Hainan was a remote and poor place in Southeast Asia at that time, and was regarded as a wild land. Demotion of officials was a punishment measure. For those who paid attention to land and acted rashly at that time, it was a great punishment to be relegated to poor places or frontiers. Hainan should be just one of them.

For example, in the Qing Dynasty, if the emperor sent you to a poor place far away from the capital, it was generally punitive, such as the snow and ice in the northern part of Taiwan Province Province (after the recovery), such as the magistrate of a county and Shengjing.