It is really tough to get murders, thieves, and thugs to give up their hostages.
[southfront.org]
I agree with @FuzzyMarineVet completely. I am also led to believe the plant also contains a large number of high-ranking NATO trainers and commanders and the Russians want them all alive.
The Russians plan to have a war crimes trial this summer and they want these mercenaries alive to testify against their handlers (Canadian General, French Colonel, several US 'specialists' and a UK weapons manufacturer representative are said to be among them)