Ingsoc85: Since the other Daedalic games are still present I assume it was the result of bad sells.
I don't buy that. If that were the case they would do what they usually do and sell it as part of an indie promo to boost sales. They commonly do that to get more people to buy certain games.
There's also the fact GOG.com hasn't officially given a reason yet. They're up to something or have been late in giving a reason why they've taken it down... which they have never been before. In fact they have often been early in giving bad news about game removals to give us time to get them such as what they did with the Guilty Gear games.
Plus I've yet to hear of a game being removed due to bad sales. All the reasons I've read about games being removed is due to legal issues or because of copyright hassles.
mondo84: There's a special edition coming out on Steam on May 6, so the original version was possibly removed a bit early before being replaced.
http://store.steampowered.com/app/268540/
I've done research and that is true but there's more. The game's been taken off Steam but a page for the special edition is now up. It says the special edition will be out May 6th. It'll be added back to the GOG catalog then hopefully.
The fact GOG.com hasn't announced any bad news about having to remove it indefinitely makes me inclined to agree with your hypothesis.