You can see how China has many population, expect more people not gamble in such country than countries that have fewer population. Another factor is religion that forbids gambling, government in such country go against gambling.

These are the two factors linked to it. I am referring to just how many people are gambling and not the sun if money they use to gamble. If it is the sum if money that is used to gamble is more in one country than the other, also you will consider how rich people in the countries are also.