Apple has a large and loyal customer base, and it's typically people who are willing to spend money on apps and games for their devices. Apple doesn't have to try and satisfy a bunch of legacy CAD vendors who don't care about consumer 3D graphics at all. And when Apple announces Metal, they don't mean they have a spec, they have an implementation that is going to have a significant installed base in under a year. They have a lot of advantages Khronos doesn't.