Perhaps not so surprisingly, individual bars of soap were once packed for shipping in a crate known as (wait for it) a soapbox. Merchants would unpack the soap for display and discard the box itself in the alley. Periodically, a passerby with public speaking on his mind would pick up the discarded container and use it to elevate himself above the crowd as he held forth on a particular topic of interest, usually religious or political in nature. This practice of standing on an actual box while speaking at length on a controversial topic eventually inspired a more metaphorical use of the term “getting on one’s soapbox”.
You have something to shout about. Hop on you own soap box here and express your opinion.