According to Facebook they couldn't come to an agreement with the IP rights owner although the same team is hard at work at a new company Vostok Games working on Survarium. I hope everything goes well for them, very disappointed that they couldn't find a way to make it work though.
From Stalkers FB "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-"
"Stalkers, while most of you are angry and upset (which we understand) please know that since December 2011 the entire Stalker team (myself included) was working without pay on Stalker 2 while new investors were scouted. That is our dedication. Sadly we could not gain legal rights to the franchise. We are still the same team working now at Vostok Games and on Survarium."
http://www.youtube.com/watch?feature=player_embedded&v=Kl812VZv9VI Vostok Games first video diary
https://www.facebook.com/officialstalker Stalkers FB
EDIT - Crap this was already posted by Phatmac, I apologize http://www.giantbomb.com/forums/general-discussion/30/stalker-2-cancelled-and-gsc-game-world-shut-down/544597/#37