To add to that: because we all know that both parties know a working money system exists, then the most likely explanation for "why" becomes "because it is profitable for the festival host." The festival goer is tight to be naturally skeptical of sny alternative payment system (smart wristbands, "credit" cards, tickets/tokens).

Even if some transactions have extra friction such as ID checks for age-restricted purchases, the less intrusive solution is color coded "dumb paper" wristbands.