If they can't just give you the money they have on hand (perhaps deposited by other customers), you are increasing the cost of the transaction for them. Cash is expensive. People complain about card transactions, but wait until you need to deal with cash deposits and withdrawals regularly - that adds up quick, and is the main reason why a lot of businesses in the UK have moved to card only since the pandemic.
It's also the case that criminals prefer cash, and a large number of small denomination notes could be used to facilitate a large number of small value crimes more easily.
Drug dealing is the obvious one, but there are others. I witnessed a shoplifters being stopped by a security guard once, with £200 worth of goods in their coat (booze, steaks), and they offered a £5 note - "it's all I have" - and by making that offer of payment, they reduced the crime and the penalty somewhat to the point the manager and security officer removed the goods, asked them to leave and told them they weren't welcome any more - no police involved. For all that bank knew, you were running a gang of shoplifters doing this.