You really aren't sure why?
It's the same reason why the iPhone 3G and original iPad didn't have a front facing camera... planned obsolescence.
Apple is a hardware company that makes most of it's profit from hardware sales. Intentionally omitting features that are widely available means that there is the opportunity to sell you an upgraded model in the future.
If you want an Apple TV, then you can buy one now that is 1080p capable, and then you can buy another one that is 4K capable in 2 years time.
You assume Apple has no competition to push against planned obsolescence though.
If you can buy a Roku 4 or 5 which will offer 4k for your 4k TV and an Apple TV costs the same and only offers 1080p still. I don't think everyone will blindly buy it, especially those who made an investment in 4k TV.