With that kind of money in the basement the theater never should’ve had to close in the first place
Ancient Rome didn't have banks or a banking system. Plus, when the barbarians come in and burn down the city, it's better to travel light on the chance that one could return when the thugs move on, and recover the cache.
Oh, duh, I just now read the excerpt -- this is a find of 5th century gold Roman coins, the theater was only built in 1870. During the demolition this was found under the old structure (that's probably why the side of the container is broken open).