There are several factors. The biggest one is that in S. Korea, e-sports is more popular than actual physical sports. Faker is more popular than Messi, Ronaldo, LeBron, etc. Which inspires people to work harder, no other region has that type of admirations for gamers. Another thing, South Korea has amazing internet and a PC Bang culture where it's normal for people to spend 8 hours a day in a PC bang and just play League of Legends all day. In other countries, that stuff is frowned upon. And since its such a tiny country, the game servers are centralized unlike in other regions like North America where if you live on one side of the country, you play with much higher ping than someone else that's closer to the servers. Everyone in Korea plays with very low ping which naturally makes them better. Places like China and Europe have several different sub-regions which fragments the player base and forces them to play with higher ping. North America only has 1 server which is stupid for such a massive continent. Lastly, e-sports is so serious in South Korea, the only way to create a League of Legends account is by entering your real life identification number and if you're a foreigner you need an ARC (alien registration card). This makes the barrier of entry to have an account much higher which deters people from trolling and making 100 different accounts, unlike in other regions.