I think it is because those big countries with good economy, they have the resources and money to buy the equipment to mine bitcoin or just to buy bitcoins. They are open minded about the change that is happening to our financial system and they are willing to venture. That is why they are becoming richer and richer, they are willing to take the risks.
that good explanation mate.usually people that have no resources they can do nothing.even for their knowledge.they prefer to give priorit to fullfill their daily need such as food for family.