From my understanding, in the short version, the main problem is that AMD bet big on HBM2 and it back fired.
See below, for more details on the long version: Some info taken from Nvidia's SC15 GPU Tech Theater
Source: LinkOn further explaining the next generation GPU architectures and efficiency, Stephen pointed out that HBM is a great memory architecture which will be implemented across Pascal and Volta chips but those chips have max bandwidth of 1.2 TB/s (Volta GPU). Moving forward, there exists a looming memory power crisis. HBM2 at 1.2 TB/s sure is great but it adds 60W to the power envelope on a standard GPU. The current implementation of HBM1 on Fiji chips adds around 25W to the chip. Moving onwards, chips with access of 2 TB/s bandwidth will increase the overall power limit on chips which will go from worse to breaking point. A chip with 2.5 TB/s HBM (2nd generation) memory will reach a 120W TDP for the memory architecture alone, a 1.5 times efficient HBM 2 architecture that outputs over 3 TB/s bandwidth will need 160W to feed the memory alone.




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