The problem with private banks issuing notes is that there would periodically be banking crises when the banks failed to pay on their notes and they couldn't pay out the gold that was demanded--that was required.

私有银行发行纸币的问题在于,当它们没法用要求的金子-规定的金子数量,支付它们发行的纸币时,它们将周期性的,陷入银行业危机。

耶鲁公开课 - 金融市场课程节选