One possibility is that this relates to China and Blizzard's substantial WoW player base in that country.

Games like Fortnite command an audience of even more players. That spot has been claimed by Final Fantasy XV. WoW is, to be sure, still a massive game - but it isn't the largest MMO any longer. It is not clear why Microsoft took a step for Blizzard that it's been unwilling to make for any other developer. Please watch out for further announcement.' In an FAQ below the announcement, the company notes: 'We are currently working with a few other game developers to port their D3D12 games to Windows 7. Microsoft also states that this isn't the game that's going to come to Windows 7 running DX12. The company has not specified which features explicitly do not work on Windows 7, but we'll undoubtedly see benchmarks comparing the two in coming days. Today, with game patch 8.1.5 for World of Warcraft: Battle for Azeroth, Blizzard becomes the first game developer to use DirectX 12 for Windows 7! Now, Windows 7 WoW gamers can run the game using DirectX 12 and enjoy a framerate boost, though the best DirectX 12 performance will always be on Windows 10, since Windows 10 contains a number of OS optimizations designed to make DirectX 12 run even faster. Microsoft writes (Opens in a new window):

Now, BfA has been upgraded several times already, most notably with significant support for multi-threading in DX12 that AMD claims can increase performance on Ryzen CPUs (Opens in a new window) (Opens in a new window) substantially over and above DX11. It's porting DX12 to Windows 7 for specific titles, starting with World of Warcraft: Battle for Azeroth. Microsoft isn't just bringing DirectX 12 to Windows 7. But not for everything and that's what makes this even stranger. It's with that in mind that I inform you of the following: Microsoft has ported DirectX 12 to Windows 7. Just as DX10 and DX11 never came to Windows XP, DX12 would never ship for Windows 7, 8, or 8.1. Other APIs, like Vulkan, might be available for machines running Windows 7 and 8, but Microsoft itself wouldn't take that step.

When Microsoft launched Windows 10, it made one thing very clear: DirectX 12 and its support for low overhead draw calls and better overall CPU utilization would be reserved strictly for Windows 10.

Windows 7 directx 12