Actually, if you look all throughout the Mafia’s history, you will see boom and bust cycles. There were decades when they were all-powerful, followed by years of not-so-powerful, and periods of being so far from power they were considered dead, to returning to far greater power than ever before. Of course in America, or really anywhere, they can't be as bold and out in the open as before. But they're not supposed to be! They're SUPPOSED to be invisible. It's supposed to be a world where those who know, know, those who don't, don't. The fact that they were as open as you mentioned is what brought them so many problems to begin with.