Today, Seoul hosted the League of Legends world championships. It's a big enough event that it occupied the outdoor soccer stadium built for the 2002 World Cup. The New York Times has a decent 40,000-foot overview of the rise of professional gaming in South Korea.
At the time, in response to the Asian financial crisis, the South Korean government focused on telecommunications and Internet infrastructure. By 2000, a vibrant community of gamers emerged, largely thanks to PC bangs that used the new connections. The clubs acted as a sort of neighborhood basketball court or soccer pitch where gamers could test their skills.
The government also became involved, creating the Korean E-Sports Association to manage e-sports. Cheap television stations took off as well, a result of the new infrastructure, and it was only natural that one, then more, would focus on e-sports.
"Fourteen years ago, you had a government that gave a thumbs-up to e-sports — it was professionally organized, and it was on television, so it became a mainstream thing," said Jonathan Beales, an e-sports commentator. "The way soccer is around the world."
You can see the opening ceremony for the LoL in the video above, and here's a photo of a wider view of the crowd from the Times:
Photo via Jean Chung for New York Times