And Apple could have made that compatible with the Apple TV 4 had they worked with Netflix.
The content is emerging - slowly but surely. There is 4K content on youtube, Vimeo etc. Not to mention that the 6S records in 4K.
At the end of the day its not like adding 4K support would have at all made the experience any worse for anyone, it just would have made the thing a lot more attractive for many. If you’ve got a 4K TV, you’re now between a rock and a hard place as we all know Apple will add 4K support with the next one, so theres no point buying one to have it superseded within two years resolution wise.
Lack of adoption/content etc has rarely been an issue with other Apple products - Thunderbolt/USB-C/USB in 1998 all spring to mind. Apple could have been the ones to help push 4K further into adoption.