It was rather obvious I thought... AMD were never going to keep paying developers to add support for Mantle long term and developers have enough tech support headaches supporting a single API (DX) let alone two. It made no sense outside of 'financial incentives' to support an API which is limited to only a small portion of an individual vendors GPU's. The reason NVidia get a lot of their (GameWorks) technology into games is because they send engineers out and do it for them at their own expense.
AMD has never paid anyone to use Mantle, developers either choose to use it or they don't.