Well, the term soapbox comes from nineteenth century England.
Back then, people in England didn't have the same rights to free speech as they do today (and certainty nothing like the ones we have in America).
However, there were certain 'free speech zones', where people were free to voice their opinions, usually on a political nature, that they couldn't ordinarily do.
To gain attention, people would climb on top of soap boxes, which were actually large crates, to be elevated above the crowd.
Today, the term means any sort of public out crying of one's opinion, especially political since this was the main use of historical soapboxes.
Thus, the term 'I'm getting off my soapbox now...' is a modern way to mark that you are at the end of an (admittedly flamboyant) voicing of your opinion.