The term originates from the days when speakers would elevate themselves by standing on a wooden crate originally used for shipment of soap, or other dry goods, from a manufacturer to a retail store.

What does sitting on a soap box mean?

informal. to express your opinions about a particular subject forcefully: She never misses the chance to get on her soapbox about political reform.

What does get down off your soap box mean?

Often, this would be a wooden box used for carrying soap in, and it became known as "preaching from a soap box". To say to someone "get off your soapbox" means to tell them to stop preaching or providing advice.