First of all, having your preferred policies enacted hardly implies anything about "the right side of history." It's just a lazy secular way of saying "God is on our side."
I'll assume you're only referring to social issues as history certainly has not trended toward the left on economics. Great attempts at carrying out far left economics have failed dramatically.
On social issues, this is essentially an exercise in confirmation/saliency bias. The endeavors favored by progressive which have failed are generally forgotten/disavowed.
*Cough* eugenics *cough *cough*
