I have had vendors tell me that they were not able to get a booth with the right kind of set-up for linking to the credit card company. Sometimes they will do credit "the old fashion way", with those clunky machines. I'm not all that comfortable with that way because your credit card number is on the receipt. If they have the really old ones with carbon, I ask that they tear it up and I watch them do it.
I also understand why the smaller companies may not take cards. The fees are awful. Their only option is to pass that on to us. I would rather pay cash and pay less.
I volunteer at a non-profit thrift store, and the credit card fees that are charged really cut into their profits.