I am fairly new to cryptocurrency so I can't say much about South Korea's Ethereum/Bitcoin craze But as a part of Steemit's KR community I think the fast growth of KR is because of those Koreans keep trying to find new way to integrate cryptocurrency into their everyday life.
For example, auction has become very popular recently (I don't know how it exactly works though) and a user @jungs even made a website called Steem market. I think it is a good way to use the system and benefit the users. Many KR users are now using it.
Also, delegation of Steem dollar to newbies has become quite popular too. Some KR whales who hold high volume of steem dollars delegated SD to users who write quality posts and bring creativity and new ideas to KR community.
I mean, these are just few examples of what KR community do with Steemit. How they work together and build system based on trust is quite remarkable.