I think there were a couple things at play.
(1) the natural propensity of these guys to keep everything secret.
(2) The possibility that some of them were legally prohibited from consorting with known felons.
(3) The fact that those arrested were carrying the modern-day equivalent of over 3 1/2 million dollars.
(4) The possibility that the attendees thought they were the target of a long term,well planned Govt. raid as opposed to Croswell's relatively quick operation.
The main motivation seems to be keeping the existence of a National Syndicate a secret, but ironically,the raid shone the spotlight on it.
At the time,the police only had suspicion of illegal activity,and had no legal grounds whatsoever to search the house.