They do better because there's no chance for the EU/NA teams to play against and practise against the Koreans outside of the Worlds, due to Riot's retarded rules. The best way to improve in anything is to play/practise with people above your skillevel, and that's not allowed in LoP.

This effect is amplified due to the 100% forced meta that Riot has created. If proteams try to play the game differently as a way to counter the eastern teams, that way of playing gets nerfed into the fucking ground. So Western teams can't outdraft or outstrategize the Koreans. This also makes it very hard for the best to get toppled. You can actually practise the exact games you'll be playing at Worlds, while in Dota you can't actually predict just what's going to happen at TI/Major LANs. Sure, some heroes are stronger in certain patches, but surprising picks can be incredibly strong as the opponent teams won't know how to play against it.