I don't understand why NV had a large market share drop after the launch of the GF6800. Belayed fallout post Geforce FX?
ATI were quicker to market with PCI-E interface variants. A lot of Nvidia's production was still geared towards AGP cards, and ATI picked up sizeable OEM orders on the back of this. Allied to this was ATI's dominant mobile market ( they owned better than 70% of the market at the time) - cards such as the Mobility Radeon 9600 Pro, 9700, 9800, and X600 dominated OEM contracts at the time. The large swings are almost certainly linked to large OEM contracts, and can say as much about the absence of hardware from one vendor as it does about the SKUs in play.
It's interesting that AMD is hurting that bad today. They have solid products selling at good prices. 960-980 must have incredible customer mind share.
Top of the mind marketing. The same reason why Intel ( who often only contribute the CPU and chipset) are often better known than the OEM/ODM machine vendors themselves. Nvidia spent some considerable time cultivating an ethos of community (for wont of a better word) amongst their buying public. ATI had much the same following in hardware but didn't look much beyond - into utilities, gaming forums, sponsorship etc. - and what they had seemed to be largely wiped away after AMD subsumed the company.
Much like the Nvidia dip in Q3 2004 - Q2 2005, I think AMD's present market share is a mixture of new arrivals from Nvidia, and a lack of the same from AMD. Price cutting might tempt some DIYers (although there are negative connotations associated with too many price cuts), but it's the OEMs that cause the real movement, and nothing drives OEM contracts like new product and new bullet points.