Ever since the 580's the incremental steps have slowed, more rebranding and less genuine steps up in power in the 2x range we were used to only years prior.
The reason I think is largely to do with GPUs becoming a hard sell on graphics alone due to the stagnation in the industry due to consoles.
This current generation of consoles was the first where we transitioned from separate development on each platform, to unified development across both platforms. The majority of the last 7 odd years has been building the same games for the same engines with little to no increase in quality.
We've seen GPUs in the PC space easily handle the console games and we've seen an increase in extras from Nvidia and AMD to help actually find a use for this wasted power, AMD pushed Eyefinity quite hard as an easy way to require 3-5x more GPU power, and Nvidia pushed PhysX and 3D to require at least 2x the power.
But even with 3D, 120hz, Multi-monitors at Full HD its nothing modern cards can't cope with easily, in fact we're already in a good position to power 4k which is 4x more pixels than HD...
I considered upgrading my old 580 recently, a few games aren't reaching 120hz on my new HD monitor, or struggling to hit 60fps on my 2560x1600 montor, after looking at my options to jump 2 generations straight into a GTX 780 for roughly 2x increase in power for £550...I simply grabbed a 2nd hand 580 and added that in SLI for pretty much the same increase but for £150
We held out hope the new consoles would breathe life back into the cycle but the disappointing GPU specs they have are worrying, they're already woefully out of date and they're not even released yet, so I'm sure we'll see a bit of improvement in the first few years of multi-platform games for the next generation consoles, but the following 5-6 years is likely to be more stagnation.
Sigh.