It's not really behind, it's different. That's very much not in the direction Apple is going, or likely to go anytime soon.
The Apple TV is intended much more as a plug-n-play set top box which can be used for various subscription services. Dolby TrueHD Atmos and DTS:X aren't available on the Apple TV because they aren't used by any subscription services. They are available for the Shield, because the Shield is a much broader scoped product.
The same goes for upscaling, anyone with a decent TV using the box for subscription services is going to be better served with the upscaling by the built-in processor tuned to their specific tv.
Meanwhile the Shield lacks some of the video support for things like 12-bit Dolby Vision or higher frame rate HDR.
My point is that if you've got a great home theater that can support advanced audio formats and a media server with remuxes from Blu-Rays with those formats, among other things (like gaming), the Shield is probably for you, but for the much larger consumer base that wants a box with a very nice and responsive UI who will probably be using it only for subscription services or for the far more common audio formats from various sources other than discs, the Apple TV is probably going to be a better product.
For me, when I want Dolby TrueHD Atmos and DTS:X, I'm going with disc anyway.