Macau is one of the most law-abiding territories in Asia. It is well known for its gambling industry, but organized crime and fixing is very rare. Most of the casinos are owned by well known companies. And perhaps that is the reason why many of the Chinese and other gamblers prefer less well known destinations such as Cambodia and Vietnam.
One of the reasons for this is they evade taxes casinos pay million in taxes, so instead of paying the government they prefer to pay corrupt government officials and the police officials so they can evade arrest, their underground operations is hard to track because the authorities that supposed to catch them are the one protecting their casinos, it's their cash cow of course.
It's not just the taxes. There are gamblers who want to launder their dirty money through the casinos (in most cases, earned from narcotic trade and human trafficking). If they gamble through the legal casinos in Macau, then the money trail will be visible to the authorities and in the long term they will land in hot soup. These people want to gamble anonymously, so that the authorities can't trace back the dirty money.