Not sure, maybe ask in the Chromecast 2015 forum, as I don't see one for the Chromecast Ultra yet.
Also remember that XDA-Developers is a user forum, not an official Google anything. If you want to provide Google with feedback, you should do so via their forums/groups.
For the original Chromecast, the answer is of course that H.265/HEVC was still in the works and distribution and availability, not to mention hardware decoders, weren't mainstream yet.
In either case, Chromecast is not really designed as a general "wireless video streaming" device, it's meant to be an extension of your existing mobile device, allowing playback of Internet-based content on your TV, offloading the work from your mobile device. It's an alternative to source device + wireless video streamer, rather than replacement.
If you want "anything I can play on my computer, I want to see on the screen" then you should look at a device designed to wirelessly mirror a screen to the TV, like a Miracast receiver or a prosumer-level wireless HDMI transmitter.