Benedict's vault is for storing Casino funds, not for long-term storage of whatever valuables people might want to put in there. He may be willing to store certain things for certain people, but only in the short term. As a gesture of kindness to the big whales that bring in lots of money to gamble with.
In actual fact, asking to store them for a long time is what would have been suspicious. As a "prominent businessman," Mr Zerga should have no trouble finding a proper bank or other secure facility who's actual job it is to store people's valuables, if he wanted to store them somewhere for a long time. Asking a casino manager to store them in the casino's vault (which is NOT its official purpose) is asking a favour, so he needs a reason that explains why he's doing this instead of going to a real bank - such as he won't be in town for long and only needs someplace safe to put them for a short time, so dealing with a bank and all their paperwork would be more hassle than he'd prefer to engage in.
As Wildcard points out below, the 24-hour limit was actually Benedict's limit, not "Zerga's." The point still remains, though, that Zerga would be acting suspiciously if he tried to insist on storing them longer.