Why doesn't Winco Foods accept credit cards?

Because any business which accepts payment with credit cards, must pay a merchant fee to the bank who issued the credit card. This merchant fee will generally be anywhere from 1% to 3% of the transaction amount.

If the business has a low profit margin, then the merchant fee can seriously cut into profits.

For example, if a business has a 5% profit margin, and the merchant fee for accepting payment by credit card is 3%, this means that the business loses over HALF of their profit.