Though originally planned to include DirectX 12 support at launch, developer Nixxes Software says it won't be quite ready come August 23. Rather, you'll have to wait a couple weeks.
"We have some extra work and optimizations to do for DX12, and we need more time to ensure we deliver a compelling experience," it writes. "Our teams are working hard to complete the final push required here though, and we expect to release DX12 support on the week of September 5th!"
Mankind Divided isn't the only high profile recent game to launch without the benefits of the new API. As such, it's not unreasonable to venture the 'closer to the metal' design may be giving some programmers trouble and that it will take time before PC developers are fully accustomed.