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''.
The act of lecturing, spouting, ranting about a particular subject to attempt to make a point or a stand. Derives from the days when people, such as ministers, actors, strikers, abolitionists, rights defenders, etc. would take a sturdy soapbox (soap came in wooden crates then), carry it to a wtreet corner, and begin preaching their point to the passerbyers.
Someone who addresses their views with an amount of passion is said to be standing on their soapbox.
In public places like London's Hyde Park individuals can advocate one cause or another. Some speakers in these public forums will bring a 'soapbox' to project their voice and to be seen by those who might cluster around.
Preaching; pontificating; voicing one's opinion loudly in the hopes it will gain supporters. Comes from a time when wooden boxes were used to ship soapflakes in, and were then put into use as pulpits for street preachers.