Cash is remarkably resistant to all the potential problems that could disable credit card transactions. For example, in 2003, much of the northeastern U.S. was affected by a power blackout that lasted over a day in some places, and of course credit card readers require power to operate. The servers that support credit card transactions have backup power (although only for a limited amount of time), but they're also vulnerable to network outages, software failures and deliberate attacks (we've seen how bad some of our major banks are at security). Having cash as a backup seems to be a prudent choice.