It's pretty expensive to take cash. First, you have to keep a float of money for change. Then you have to secure the cash until you deposit it in the bank. Then sink some time into counting and depositing it, before being charged by your bank for depositing cash. That must very quickly add up to a percentage point or 2.
The issue is that these costs of handling cash are largely fixed - marginally, if your store takes $100 in cash, your cash-handling expenses and effort stay the same, but if you take $100 by card, your fees directly increase. You avoid the expenses of handling cash only if you don't take cash at all.
