I asked for a lot of £5 (smallest denomination note in UK) at the bank once because I was going to a music festival (before mobile card payment was routine). They asked me casually why I wanted them and I explained. Seems to be a Know Your Customer thing.
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.
In my jurisdiction it is enough if it is evident that you intent to steal something. Theft can only be prosecuted if there is intent. Usually if they suspect someone shoplifting they wait and spring the trap when there are enough clues, for example someone briskly walking to the exit.
One story: My wife got stopped in a departement store because she took an item in a sub-shop in the underground floor. It's not a mall. Then she went upstairs to look for more things to shop. The police was there after a short time. This means the shoplifting surveillance system has already sent out an alert to the police when my wife was on the stairs, because employees aren't allowed to hold people. But my wife didn't walk briskly to the exit but strolled around and when stopped she told them she wanted to pay for all things together. The police people even laughed and the employee directed my wife back downstairs to pay.
The banks often have the $2 notes, but you have to ask explicitly.
Although who knows if that's the whole reason?
Edit: digging up a citation suggests that it might be a voluntary code that banks are signed up to [1], but the regulatory regime has still swung much more in favor of the customer.
[1] https://www.citizensadvice.org.uk/debt-and-money/banking/ban...
If you watched Kitboga videos on Youtube, you know scammers often ask for gift cards as payment.