I think its just a matter of Apple wanting to innovate faster than web standards body will allow. They did this with Metal when OpenGL/CL was lagging behind in some critical performance improvements. In the case of tvOS, I think the innovation is in the user interaction model. Apple has shown to contribute ideas back to standards bodys though (Vulcan, USB-C, etc). I think when web standards catch up to a well-defined TV-based user interaction model, webkit will be included in tvOS.

I can understand the frustration though and the loss of OAuth. I wonder who many will (already?) support JWT.