As it says it looks as though the war against the Finns had become too costly for Stalin who needed the men elsewhere, to fight the Germans. He could not fully achieve his objectives with the amount of troops he could afford to spend against the Finns and in the time he had to do it, so he had to drop the request for unconditional surrender.

It looks as though Stalin could never learn from past mistake's in the Winter war when he got a bloody nose with twice that many men.