I think there is no quick answer on this question and as long as somebody has worked at Intel and investigated this issue you can never be sure if the explanation is correct or not.
I can only guess but I think the reason of a cold bug is related to the basic materials used and process parameters. Every generation has a different basic wafer in terms of silicon doping, different alloys deployed on top of it and so on.
With a decreasing temperature the resistance of silicon is increasing. It could be that at a certain point some parts are just not working anymore.