I suspect that when you're talking about a facility that hosts 70,000 people, it is substantially more efficient. How many points of sale are there in that dome? Hundreds? If each has to accommodate cash, that means that each has to: have some allotment of cash to open with, have a place to keep and secure the cash during the event, and have a process to close the point of sale and deliver the cash back to the central accounting operation.
There's no way around doing any of those things if you're going to accept cash. Going cashless eliminates each of those requirements. It's incontrovertibly more efficient.