Mantle was an AMD API which was replaced by the Vulcan API, which is a collaboration between several industry leading technology companies. Calling DX12 (a Microsoft API, mind you) an AMD standard is plain wrong.
Going async is a natural evolution of software, as well as hardware. It wasn't supported at all in DX11. AMD has been working on their GCN architecture for years, where it's supported natively. This in other words meant that their cards have been under-performing due to their worse single thread performance. Until very recently, all games were developed with DX11. Nvidia went another direction, focusing on what API was primarily used, and thus, most of their work has been optimizing their brute single-core strength.