Lower TDP, better stock cooler (blower that doesn't heat up other components - remember heat is electronics worst enemy), less capable PSU required, those might not be important factors to computer nerds but OEM's like Dell who buy in bulk love it.
It all comes to R&D in the end, AMD might still be able to compete on performance alone but both Intel & NVidia who have been criticised for holding back performance have been working tirelessly on reducing manufacturing costs (ie. making products smaller and more efficient) and are now reaping the rewards.
GTX780Ti = 7.1bn transistors/561 mm².
GTX970/980 = 5.2bn transistors/398 mm².
Yet GTX970/980 are clearly superior and have only a 256bit memory bus (cheaper PCB also).
The GK110 has a lot of transistors dedicated towards compute and even the wide memory controller is there for GPGPU workloads.
Its why Nvidia has just slightly rejigged the GK110 as the GK210 and kept it as their top tier GPU.
The Maxwell GPUs have a lot of this functionality removed which helps towards making them smaller and more efficient.
The AMD GPUs are not that much bigger than the Nvidia ones,considering their dual uses. However,the main issue is that top end AMD cards have wide memory controllers,which means added costs,but like the GK110 they were made to serve dual uses.
The GK110 after all was a 565MM2 GPU competing with a 438MM2,but Hawaii is now old,so its competing with tech which is much newer,and against GPUs not made for mixed workloads.
If you even look at the workstation versions of Hawaii they fit into lower TDPs but are clocked lower. I suspect the desktop gaming versions are clocked a higher than they should for optimal performance/watt.
The only realistic way AMD needs to address this is to just expand their midrange chips,and strip out more of the unneeded functionality.
However,looking at Tonga,in some ways despite the reduction in DP compute,its unusually fast for OpenCL stuff,which indicates Apple might have had some involvement in drawing up the specification IMHO.
But they really need to do something especially for mobile - that is probably a massive issue too,and even more than desktop.