According to official news hackers from North Korea have stolen in the last 6 years a total of 3 billion dollars in cryptocurrencies.
It is a huge amount of money for the emerging economy of the communist country.
They hacked the U.S. but it looks like they initially targeted the South Korean exchanges, and since they couldn't hack a lot of money, they decided to exchange the target for another country.

And since they couldn't hack a lot of money, they decided to exchange the target for another country.