I blame the shift in winrates on players getting better or worse.
For almost a year, only MC, Genius and Huk were actually any good in Korea/World, and Naniwa was only good outside Korea.
Now we've got Oz, HerO, Parting, and JYP.
On the Zerg front, Nestea and Leenock have been slumping, so they only have DRG in Korea now.
On the Terran front, they always had the most players and more good players, so MVP slumping is made up for by guys like aLive, Polt, and MKP coming back.
In Korea, there was always a racial imbalance because the majority of the good players that switched to SC2 picked Terran due to Terran being perceived to be the strongest, and because Terran seemed to benefit the most from having high APM. The influx of talent to Terran only further increased the perception of it being better since there ended up being a talent imbalance.
Protoss definitely got the shaft in terms of talent switching over. Protoss had the fewest successful and star players since the beginning, and only recently has that changed. Unless Genius wins tonight, MC remains the only Protoss to win GSL after 1.5 years because MC is the standout talent of the Protoss.