>>718432001 (OP)
Because of Korean autism who had that shit on television by mid 00s. In the West, while popular, it really didn't have a proper esports infrastructure. Mostly it was limited to few annual big tournaments that might've had qualifications for them, such as World Cyber Games and Blizzcon, but Warcraft 3 also had these.
As to why SCBW popped off in South Korea that early, I do not know. Of course it was popular in net cafes and what-not, but so were a lot of other games. It's quite amazing how they had a proper league system set up with multiple teams (some sponsored by massive corporations such as SK Telecom and STX), and these teams also had their own in-house feeder teams where the best "B-players" would compete for spots in the A-team. Even Korean Air Force had their own Starcraft team, full of early 00s SCBW pros who couldn't delay their mandatory military service anymore. In 2010 they had a tournament with Korean Air as the main sponsor, and the finals between Flash and Effort were held in a airplane hangar. I think it's safe to say that it had gone mainstream by that point.
I'm not sure Blizzard would've invested that much money into SC2's esports had it not been this popular in South Korea. It's pretty funny that early SC2's best Koreans were often players who had failed to climb from this aforementioned feeder team system.