The reason we cannot continue with the development of Stalker 2 is we and our new investors we unable to come to an agreement with the IP rights onwer. The entire team that was working on Stalker is now working on a new project called Survarium. We are so grateful for the support over the years and despite our best efforts we have to move on from Stalker which is as hard for us as it is for you. However, we feel that our new studio and project will be something unique and fill the void left by Stalker as well as add a lot more! -Your Stalker Team-
Here is a video they posted talking about gsc closing down.