20.2% in July, 18.6% in August, and 17.1% in September...You can blame AMD, though it isn't really AMD's fault, it's the miners keeping prices so heavily inflated. The RX 580, for example, is still a good $80 over launch price. You can blame Vega's issues, but Vega is high end and high end cards are a small percentage of the market, it's the mid range like the RX 580 and GTX 1060 which are the bulk.
People are wondering why developers aren't taking advantage of AMD specific features. What incentive do they have when their market is 3:1 in favor of nVidia? And this is on top of the rather intimate relationship nVidia has with the major game studios.