It surprises me that relatively small countries are in the top 10, which means that not only economic power is a factor, but also something strongly cultural.

Being a Spaniard myself seeing my country there doesn't shock me especially, because I know that we have a great tradition of gambling although there are not many famous casinos in here (EuroVegas project failed before even starting), so I think that for many people it can be counterintuitive. Just like a few more in that list, but the most obvious ones are also there, so it must be right.