It’s a part of the legacy of Puritanism. Someone on medium wrote a great series tying modern day progressivism to its roots in Puritanism. Seems to me it was Tobias Stone. The Puritans are still around, they became the Unitarians. The Unitarian Church (formerly Puritan) is the defacto home of Progressivism.
One of the main traits of Protestantism is renunciation of sin —it’s the only path to salvation for Protestants. We are seeing the secularization of Christian renunciation of sin. Of course in all forms of Protestantism the individual is responsible for professing their sins to Christ. I guess in progressivism renunciation of sin is critical to social purity.