It has nothing to do with poverty: even in the most booming 1990 there never were diadems at the Red Cross Ball. It is simply all about the dresscode set by the organization (La Croix-Rouge Monégasque): cravate noir, black tie.
Sometimes the dresscode even is "The Roaring Twenties" or "Pirates". It is no official State Event or something like that, just a fancy fundraiser.