A lot of small vendors and publishers either can't take your card or offer discounts for cash. It always seems like these are the vendors that have the book or trinket you want the most. Also, take your checkbook. While many of these vendors can't take your card, they will often take a check.
Another reason to take cash is that it makes getting meals a lot faster and easier, especially with big tables with multiple payment methods. This is almost guaranteed to get you to your game or event on time.
Lastly, quite a few small publishers run their own games and have some copies for sale afterwards, and cash makes that a hell of a lot easier.