With digital payments you see exactly how much the payments cost. Swipe fee + processing fee, and it isn't fun as a business owner to see ~3% of your money going to a bank.
With cash, it takes some effort to tally the costs. Cash handling equipment: POS, safes, bill counters for larger merchants, etc... Then you have to add the amount of time your employees spend on cash: counting drawers at the start, counting drawers at the end, managers counting money into and out of the safe, managers taking cash to the bank, or hiring a service to do that, cash goes missing through malice and negligence, etc...