he addressed in presentation, from what i remember it had a lot to do with cost. Like $130k/ton with carbon vs $3k/ton with steel. He mentioned a few other factors. Also it allowed them to build outside, not sure if t that is true in production.
You realize that all bitcoin transactions can be tracked. People have sent BTC from their wallets to Quadriga, it should be trivial to find the cold wallet address. Should be easy for anyone who understands blockchain forensics.