Because there hasn't been any technological advancements in GPU for over 4 years! We are also stuck on 28nm technology for the past 4 years and in the way GPU's rely on smaller node in order to improve performance and/or cut power consumption we haven't had it.
Also at smaller node you do more on a smaller size, that means production costs decrease, sure at first it might be even more expensive until the process is mature and stable, but very soon smaller node pays off.
And again, with zero technological advancements, still stuck on 28nm, Nvidia getting more and more marketshare they are more and more acting like a monopoly and keeping prices high.
Just look at the GTX 960 2GB which released for $200, a useless 2GB turd that is barely mid level for $200, 4GB versions cost $30 or $40 more.
Even the latest Nvidia turd the GTX 950 is $160, with custom versions going for mostly $170 and $180. So you have a low end turd going for $160 in the minimum, that is Nvidia for you! And since those who buy Nvidia are actually mentally not with all the fibers in their brain they'll keep on buying these useless overprices turds!
Fortunately the wider market is smarter in general and has stopped buying graphic cards leading to record low sales, and again why would anyone buy GPU's in this market when its filled with overpriced turds that have the same performance as GPU's from 4 years ago?
So a combinations of zero technological advancement, stuck on 28nm and Nvidia being a garbage, lying, stealing, cheating, scamming company and releasing useless overpriced turd after another that their worshipers keep on buying and you still have $300 mid range GPU's being sold as "flagships"!
Just look at Nvidia with the GTX 980, they released the GTX 980 at $650 as the FAKE flagship product and they gave it in their behinds to all those who were foolish enough to buy a FAKE flagship. After they scammed the idiots out of their $650 for a second tier GPU, they released the REAL flagship the Titan X for $1000. All the Nvidia worshipers bought it as the next best things since sliced bread and then Nvidia roped them in their behinds as well by releasing the GTX 980ti which was actually mildly faster than the Titan X at $650.
Of course the GTX 980ti being $650 is a scam in the first place, flagship graphic cards have always been around $500, we've never really had overpriced turds until the first OVERPRICED TURD the GTX TITAN. That was the first overpriced turd at $1000.