Because there is a dramatic reduction in costs from going completely cashless. You don't have to pay for security for cash-on-hand. You don't have to pay for an armored car to transport cash to a bank. If you don't want to pay for an armored car, you don't have to pay (and trust) an employee to do it for you, else paying with your time by doing it yourself. You can serve more customers in less time because you no longer have to make change with each transaction, and you no longer have the cognitive load of ensuring that the bank gives you enough change and in the correct amount on each bank trip so that you can make change for your customers. Because you don't need to trust your employees with cash anymore, you have reduced the risk associated with the hiring of an individual employee, which makes it easier to hire ex-convicts - and in many areas, that (unfortunately) makes it dramatically easier to hire people, period.
Sure, these costs are balanced out by the formal costs associated with card processing fees and no longer being able to partially hide income from the taxman. But the flip side is the improvement in trust between management and labor, which overall ought to make cashless businesses better workplaces.