OED lists soap-box

1650s, "box for holding soap," later especially a wooden crate in which soap may be packed; from soap (n.) + box (n.). Typical of a makeshift stand for a public orator since at least 1907.

Or from Merriam-Webster:

: a box or small platform that someone stands on to give a speech to people in an outdoor, public place

: an improvised platform used by a self-appointed, spontaneous, or informal orator; broadly : something that provides an outlet for delivering opinions

Basically, when we refer to "soapboxing" as an activity on /r/AskHistorians, we are referring to those who are attempting to use the sub or history as a platform to promote an agenda, be it political, religious, social, etc. These can be in the form of a question (i.e. a loaded question) or an answer that seeks to use the past as a pretense to steer the discussion towards a particular issue or debate topic. From the Rules:

This subreddit is called AskHistorians, not LectureHistorians or DebateHistorians. While we appreciate your enthusiasm for the history of issues that play a role in your life, we are here to answer your questions about issues, not provide a sounding board for your theories or a podium for your lectures. All questions must allow a back-and-forth dialogue based on the desire to gain further information, and not be predicated on a false and loaded premise in order to push an agenda.

This corresponds with a further section in the Rules:

Answers should not include a political agenda, nor moralise about the issue at hand. This is not the place for you to say that communism is a failure and against human nature, nor that capitalism is evil and dehumanises people. Historians report the facts and events as neutrally as possible, without an agenda - moral or political.

In other words, we want to learn about the past on its own terms, not push an agenda. Soapboxes must be left at the door :)

Ah okay! I understand Then, Thanks for the explanation.