Holy crap, I just looked at the Radeon 580s prices out there. Around $500! Are Radeons that much better for mining? No wonder they are not showing up here.
My understanding is that Nvidia chose to gut integer hashing performance on most (if not all?) of their consumer GPUs a few years back. Instead, they directed their resources towards floating-points calculations, which are much more relevant for gaming.
AMD/ATI did not make such a concession (pretty sure GCN architecture actually improved integer calculations), and their cards continue to handle integer calculations well.
That said, miners are certainly NOT the audience for these consumer/gaming GPUs, as new iterations simply do not target things that matter to miners. Improved hashing performance and energy efficiency (in terms of mining) are not a promise when Nvidia or ATI announce a successor. For example, there's no guarantee that a future 680 will achieve as many hashes per second as a 580 while mining, because like Nvidia, hashing performance does not matter to AMD when it comes to gaming GPUs. However, even if a new card does achieve greater hashing power, it must still must remain energy efficient. A theoretical 680x could be an absolute monster, doubling the amount of hashes per second, but that gain is near worthless (for most) if it increases power draw (and energy costs) by 4x.
Anyway, I've digressed quite a bit here! The cost of 580s is absolutely out of control, and cryptomining is definitely driving demand even higher. Even the 380 and 480 are being sold USED at a premium. Fortunately it seems like AMD is finally launching a line of cards specifically for miners that will hopefully help alleviate the apparent shortage of mid-range consumer/gaming GPUs.
Edit: Forgot to mention that ATI cards also tend to have lower MSRPs than their Nvidia counterparts, which is just icing on the cake (assuming you could get one at that price, lol)