The devs have stated that they will be bringing the prologue to GoG, Steam, and all the other storefronts sometime soon. No official release date has been announced. The reason being is that the prologue and the main game are built in different engines. So instead of porting over the prologue to whatever engine they are currently using, they chose to instead polish what they had with the main game.

I did find it odd that the game keeps referencing things in the prologue when a good majority of people do not have the prologue at this time. It seems a poor idea to have your demo available only to the early backer crowd and make others wait for it later on. If they knew it was going to be a hassle to port to a new engine, I think they should have removed the references to it. Then I would not have a feeling of jumping in a series at the middle rather than at the start.