Asia is too big, and everything there is big, so does gambling industry. Macau is bigger than all other gambling cities, and they wager more money there then anywhere else in the world.
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.
After Las Vegas, Macau is the most visited place for gambling. If not mistaken even gambling constitute to 85% contribution in their economy. So it is all gambling and the tourist that comes to visit Macau. Also, people who cannot afford to travel Vegas, ensure that they do visit Macau to enjoy the gambling and nightlife out there. Since China and Macau are quite close by so people do visit on Friday night and then go back to China on Sunday.