Not baffling at all. PS4, Xbox One, Xbox 360, PC = all DirectX.
OSX, Linux, etc...tiny, tiny market share in comparison. Android/iOS, most people are hitting a middleware platform like Marmalade/Unity/UE4.
Sometimes a single proprietary API is a better way to move forward in a fast paced technology space. There's a reason Flash, Java, Silverlight are still lingering on the web, it's because open standards evolve at a snails pace.
See :
http://arstechnica.com/information-...released-with-one-of-direct3ds-best-features/
Plus DirectX 12 is going to offer lower-level hardware access a la Mantle, on a
hardware agnostic API.