I'm not as informed as most on here but my guess would be the combination of gambling and loansharking. The two go hand in hand and with loans a mobster can then get into the legit business of the debtor and from there. Get into another connected business. Also it probably depends on the individual capabilities of each mobster. You can be a terrible bookie and lose your as s the same way a person could be a crappy business man. But the best can and do make fortunes on any industry.