It because those countries are have good technology and they know how to spread information,and they have enough money to manage the bitcoin.
Their government can understand that bitcoin is a valuable currency for their citizens and also for country's economy increasing that's why they want to improve bitcoin currency and people can easily earn and invest in bitcoin, if other countries where government cann't accept this currency they don't know it will not good for them because their people will not become rich immediately and their life will suffering from many problems, so every country should accept this currency for their people and their country's economy.