In addition to tax-dodging, or selling off-the-books, there is one other, small legitimate reason for a vendor, to avoid card payments:
Most banks/payment processors would charge a fixed amount per small transaction. Amount varies a fair bit, but as an order of magnitute, say - €0.15 or something like that. This means customer that bought cup of coffee for €1.50 and insists on paying with a card, will cost vendor whooping 10% extra (it is not allowed for vendor to pass on card transaction cost to customer).
For a larger purchase, €30, €60, and more - transaction charge is negligible, naturally.