Why do you think that could be?
To stop these cards being sold so quickly to mining customers.
Sure, a card sold is a card sold - it makes the same profit no matter who buys it.
But a gaming customer is so much better for the company!
1) There's a big issue with guarantees: in some countries consumer products have a minimal length of guarantee, e.g. 2 years in most of Europe). Gaming GPUs aren't built to survive that ~17500 h of heavy load, so companies would have to increase production cost.
This could be, among other things, the reason why vendors seem to ship more high-end products lately (e.g. ASUS Strix, MSI Gaming and Gigabyte G1 are way easier to find than the lower-end models).
If GPUs are divided into 2 segments: consumer (gaming, home GPGPU) and commerial (mining), the latter one could be sold with much shorter guarantee period (the NVIDIA is rumored to offer 3 months).
2) Gaming (and GPGPU) business is pretty simply and easy to forecast. This makes investing in gaming hardware pretty straightforward and low-risk.
Developing a new GPU (or CPU for that matter) takes years. We know N millions people are gaming today, so it's very likely that N millions will game 3-5 years from now.
How many people will be mining in 2020? Will mining still exist? Which financial institution will support developing your new mining-oriented products if you might not sell it at all?
Also, I don't think gaming crowd would like such a change of the market. We're not far from a situation when only miners (commercial users) can afford new GPUs, while gamers have to buy used parts. This would make the GPU business strangely similar to automotive one...