It's probably because AMD is able to release it's full lineup in the space of 2-3 months, Nvidia seems to have problems even getting the high end out, so takes ages to release the mid to lower end parts.
Thats AN issue, but not the main issue. Nvidia's by FAR best selling cards right now are the GT310-340 cards, they've been out since last August or so, it doesn't help they are DX10.1(and I believe the 330 is DX10, though maybe only in the mobile segment), but they are actually VERY small cores purely because they have so few shaders, so little "newer" functionality. They've not actually been able to make a 4xx card small enough, the 430GT is 96 shaders, they need something with 16-32 shaders and 1/3 the side, but the design(based off GF104) means 48 shaders per cluster, and just in normal design a pair of clusters means 96 shader cores are VERY expensive to put in $30 cards, hence the 310-340GT still being their main cards for selling to the likes of Dell/HP, etc.
The problem is they are losing those contracts left right and centre, simply because AMD offer the same things at lower prices, or in the low end, superior features now.
The GT430 is slower, more expensive, with lower IQ than a 5670, its slower most of the time with lower IQ and costs WAY more than a 5570, so Dell choose AMD cards in the low midrange segment and thats where a HUGE amount of profit and sales are.
Apple were basically exclusively Nvidia and are pushing faster towards being exclusively AMD(and Intel intergrated). Dell were almost 100% Nvidia and now no where near that, it adds up, fast.
But most of these guys work off contracts, so computers designed and tested middle of last year to use GT310's are still being sold, but the computers designed now are ever more likely to be designed around AMD options, almost more than anything else because they can trust AMD when they give them a date for a new GPU and Nvidia are so late those guys can't plan their ranges properly.
Its not about performance or fastest single card to Dell/HP/etc, its the few cents saved a card, feature checklists, lower power, and providing things on time that win, and those guys make up the massive majority of Nvidia/AMD sales, end user buying their own cards is sub 1% of their revenue.
Basically till Nvidia can prove they can be competitive, power efficient, as fast as the competition at all price points, cheaper and prove they can deliver on time they'll be taken less seriously year on year.
