You have to think back to the community we are talking about. If we're talking about the beginning of the the mafia we're talking about Italian immigrants in the early 1900's in the same neighborhood. These people didn't have TVs or Radios and actually talked to their neighbors. If someone had a gambling ring set up or was selling goods at a lower price, word got around.

It's not like these days where no one knows their neighbors or what's going on in the community.