There's a small chain of breakfast/lunch restaurants in Pittsburg that we frequent for breakfast. They only take cash. They have directions to the nearest ATM posted. I don't know why they don't take cards. Their prices are about the same as every other restaurant that does. Mystery to me.
There's a restaurant in New Buffalo, Michigan that doesn't take credit cards. Only cash. There's an ATM in their lobby. I don't know why, as their prices are kinda high for what you get. Pretty good food, so I'm not complaining, but it can't be a financial reason.
There's a restaurant in Mishawaka, IN that also only accepts cash. Again, it's not because of the cost.
None are suffering for business and are always busy.