There are a couple of reasons behind this. The biggest reason is the availability. When AMD launched Radeon HD 7970, they didn’t have much stock to offer but right before NVIDIA GTX 680 release a lot of HD 7970 cards miraculously started to appear but NVIDIA’s GTX 680 is out-of-stock from every shop. So, this still renders the HD 7970 to be fastest GPU ‘available’ right now and people are still paying 50 bucks extra for it cause they don’t have any other choice.
Another problem AMD has is their current pricing scheme which stems directly from somewhat unaggressive pricing. With $100 separating every HD 7900, HD 7800 and HD 7700-series card (with the exception of the HD 7750), reducing the price of one would have a cascade effect upon the entire lineup. For example, dropping the HD 7970 by $50 would effectively render the HD 7950 overpriced, knocking the Tahiti Pro-based card down by $50 would have an adverse effect upon the HD 7870’s sales and so on. It looks more and more like NVIDIA has pushed the high end Radeon lineup into an untenable position for the foreseeable future and with Kepler poised to make the jump into lower price points, things could get ugly if AMD tows the same line.
Source: Hardware Canucks