The short answer is that they don't want to make it easy for banks to hold cash; they want the banks to loan the money out (ideally) or deposit it with the Euro

The short answer is that they don't want to make it easy for banks to hold cash; they want the banks to loan the money out (ideally) or deposit it with the European Central Bank where they will have to pay for the privilege in the form of negative interest rates (which is what the banks are trying to avoid by holding the cash).