According to Russian reports, STALKER creator and GSC boss Sergei Grigorovich has issued a statement revealing that the reason his team couldn't get the rights to the STALKER series in order to make a sequel — as only briefly alluded to earlier today — was because those rights have been sold to Bethesda.
"I realised that this concept is the stalker will not be of interest to many players and decided to remove all the work", Grigorovich apparently said (via Google Translator) on the site. "Announcement will not be shown instead, all that the team managed to do without me. STALKER brand into the hands of Bethesda games. I'm leaving the game development — fed up with..."
Note that the comments appear on a Russian fansite, and have in no way been confirmed as having been actually issued by Grigorovich.
Reached for confirmation, a Bethesda spokesperson declined to comment on the matter.
S.T.A.L.K.E.R. 2. Новости. Продолжение [Stalker-GSC]