It's been a hit and miss with the AMD Radeon RX Vega announcements followed by it's launch two weeks ago. Pretty much on launch day the prices went up, and stock did dry out fast as there likely wasn't a lot of GPUs available.
In a report on Digitimes they now claim that the shortages will last at least until October, the reason behind this would be the low-yields of the GPU packaging. Likely (and I can merely speculate this), the culprit would be HBM2 assembly onto the die (as the memory sits on the chip and not the PCB).
Next to that, Vega10 is a fairly sizable chip to fab, and that often shows in wafer yields as well. AMD on their end mentions that the demand is bigger than expected and that they will be working with partners closely to get demand and supply under control.
Digitimes - Some sources pointed out that Vega's design of integrating high bandwidth memory (HBM) into the GPU has significantly raised the difficulty of the related packaging process and hence resulted in low yield rates. However, some other sources claimed that the issue may be down to problems with Advanced Semiconductor Engineering's (ASE) packaging technology. Commenting on the reported shortages, AMD said that Radeon RX Vega64 demand continues to exceed expectations and the company is working closely with its partners to address this demand. AMD is also working with its partners to restock all SKUs of Radeon RX Vega64 including the standalone cards and Gamer Packs over the next few weeks, and consumers should expect quantities of Vega to start arriving in the coming days.
AMD's Radeon RX Vega GPUs are currently manufactured via GlobalFoundries' 14nm FinFET manufacturing process and are integrated with HBM2 memory from SK Hynix and Samsung Electronics. The packaging is handled by ASE using the SiP technology. Because of the shortages, most graphics card vendors are only able to receive limited supply of the Vega GPUs and the shortages could last until October.
With the challenge from the Vega GPUs not as big as expected, Nvidia is also not in a rush to begin mass shipments for its Volta-based GPUs at the end of 2017 and has rescheduled them for the first quarter of 2018 based on Vega's status, according to industry sources.
As you can see, Digitimes also mentions that Nvidia Volta chips might have been delayed from late 2017 towards Q1 2018. here again I can only speculate that in that case the reason would be the lack of competition. You can read our reviews on Vega here (Radeon RX Vega 56 / Radeon RX Vega 64).