In the days of early politics after 1776, long before television and radio, political candidates would stand on wooden boxes, often ''soapboxes'', in the town square in order to address large crowds. The term soapbox stuck, and ever since then has been used in instances when somebody tries to preach his or her personal views.
There goes Mary trying to tell everyone how to live, I think I'll tell her , ''Get down off your soapbox''.