How religious the country is will also determine their gambling habits. It doesn't matter how wealthy the people over there are or how vibrant their economy is, if the country has more religious population their gambling activities will be extremely low compared with countries that doesn't care much about religion. So this should also be put into consideration when deciding why some countries gambles more than others.
You are right, In a lot of Islam dominated countries gambling doesn't seem to be something that's done by a lot of people because of their religion. This is the reason why some countries gamble more than others. But in a country where religion reduces gambling activities there are people that would still gamble, just like I said it's only reduced it can't be stopped because some people love doing it