Money.  ~$10M/day is made mining ETH now.  A year ago it was ~$1M/day, and 15 months ago it was <$500K/day.
Bitmain's ethash miner is not going to have a significant impact, because unlike bitcoin and sia mining, it requires large amounts of RAM.  The costs of the RAM significantly exceeds the costs of the ASICs.
At US$450, ETH is now 1/3 of it's peak in Jan, and that is what is having the biggest impact on mining and GPU demand.  Summer is coming in the Norther Hemisphere, which means miners with expensive electricity costs will be shutting down rigs.  Unless the price of ETH climbs back over $1000, I predict the GPU shortage will be over by mid-summer, and GPU cards will be readily available at their MSRP.