My speculation: 1C yanked the game from the shelf (perhaps others as well) so it can put money into the HD remake, and then sell that. I am not sure how/if GOG will make this game available to players, especially those of us who already shelled out dough for the existing version. Personally, I'd pay a couple bucks for an HD patch... but only a couple.

I'd just like to clarify here that current owners of the previously sold GOG version can still download it. I probably purchased it on a sale and forgot that I had it. When I was getting around to actually trying to play it, I was wondering why I couldn't click on the title hyperlink, which would usually direct you to its store page. I downloaded it just now though and verified that it works.