SC2 is in decline everywhere and in Korea, whilst League of Legends has exploded and is the new new e-sport king. 25% of the population plays it afterall, which SC2 is not even remotely close to.
The thing with SC2 is not only is it extremely slow in balance changes (Riot Games with LoL on the other hand are making too many changes per patch
), it's also not very easy on casual players nor is it very fun to watch these days where the metagame is turtle up to 3+ bases and then have one big battle that decides the match instead of multiple small skirmishes at different places on the map like Brood War was. The main culprit is the Zerg Broodlord + Infestor composition which encourages turtle play. Just look at PvZ... it's decided by some lame 2 base immortal + sentry all-in or Brood Lord + Infestor vs Mothership Vortex and the game is won or lost by a single vortex. Not very interesting from a spectator point of view.