Part of why cashless works is the same reason the Swedish welfare state generally works: a trust of government. Contrast this with Germany and its history with...lists; it's best not to have yourself be too traceable. I think the US is somewhere in-between, but at the end of the day, we'll hand over a lot for 2% cashback and avoid paying for journalism at all costs.
Another consideration is PoS cost. In Denmark, I know that credit card PoS's are super expensive to rent. And you need to rent them, because if you buy them - which is even more expensive - you'll have to replace them every year. So you need a certain amount of turnover to have a credit card PoS in Denmark. The card PoS's come in two forms; one that only takes the Danish Dankort (which is the cheaper) and one that al…
Also, the Denmark-only Dankort is dying and its demise is happening fast: they are coupled as Dankort/Visa cards and the POS might decide to prefer Dankort but accepts Visa just as well. Neither Google Pay nor Fitbit Pay support Dankort and I have not seen transactions fail that would work with my Dankort.
Also, cash transactions are pretty rare too. Even older people use contactless, I have never seen cashiers have to pick coins from someone's hand as is customary with older people in Germany. Sometimes days pass without me seeing anyone pay with cash. In my experience the main source of cash transactions are German tourists.