My sister had a booth at a quilting expo a few months ago. It was a one-time affair and she said the fees to do credit cards (along with the additional cost to get power and a phone line to her booth) would have not been cost effective. She told customers that she would ship to them if they wanted to go to her website and use PAYPAL if they did not want to pay cash or write a check. Only a few got upset that she wouldn't take a credit card. She ended up selling out by the end of the show on Saturday, despite not accepting credit.
I think the smaller expos attract more "Mom and Pop" type booths that cannot afford the fees for credit cards. The larger expos (that charge upwards of $700 to rent a small booth) probably all take credit cards.
I am going to one of the big ones here in Denver next month and I am definitely taking all three... just in case.