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Because the HD 6850, based on obsolete VLIW architecture from 2008 (albeit refined, but still based on it), is in legacy status and will receive no more driver updates, and the sub $100 RX 460 has near 25% more power than the 6850 while drawing near half the power.
Matter is not in drivers support. HD 6xxx (and 5xxx) doesn't meet minimum system requirements of VULKAN or DX12 because of lack of some hardware capabilities.
It is decision of Microsoft and Khronos Group.
The old HD 5000 and HD 6000 series cards are in legacy. They work fine with DX11 titles but to not expect miracles.
Vulcan is barely a year old and antique cards lack enough GFLOPS to get the job done
GCN cards like my ASUS HD7870-DC2-2GD5-V2 » HARDCORE GAMES™
have more of a general purpose architecture which makes it easier to implement disparate APIs but even this will have it tough looking forward
once the second gen 14nm cards come to market, DX12 should be firmed up better