Metal as a whole but especially aluminum transfers heat really well the way to stop that is to create a vacuum between the walls of the cup which costs significantly more than plastic forming, then that vacuum can be readily damaged if it’s dropped and it’s even recommended that it isn’t put through heavy dishwasher cycles which both are likely to occur. Finally you have the issue that people would just keep them for home as the double walled cups can be expensive which would mean they aren’t available in shops and thus you have to use more single use plastics. While plastic cups don’t sound great this plan would reduce overall plastic usage