Because NVidia has a program called Gameworks, which developers can sign up for to optimise their game for NVidia cards – 60% of the market. The closed and proprietary Gameworks makes it almost impossible to optimse for AMD, which means that $500,- AMD cards perform worse than $300,- NVidia cards, unless the developer has spent a huge time and investment from the start of the project to optimise for both brands. AMD can’t fix this with a driver update, because reverse engineering the NVidia Gamesworks is difficult and the system is filled with obscure cruft and worthless junk, making it difficult to find out how to optimise the driver for it.

Why 'Watch Dogs' Is Bad News For AMD Users — And Potentially The Entire PC Gaming Ecosystem – Forbes.