Just because they decided to drop the Web UI concepts do not mean that what they chose was more powerful or the better decision in general -- it suggests it was the right decision
for them.
Also, since when was Google "the" company pushing web UIs forward? Mozilla is that company in my mind -- remember the time when they made a whole mobile operating system based on it, and at the same time tried to push forwards for various parts along (WebBluetooth, other APIs)? That's what investing in the web as a platform looks like. Flutter is successful, but not in a way that plays nicely with anything else -- if you want to push a platform forward or encourage growth, doing things that play nicely with others even if they're less convenient for you is the way to go.
I'd love to be enlightened to what you think is the problem with DOM in particular that makes it not the right model -- TVML and UWP dropped the DOM, but basically replaced them with their own walled-garden re-implementations of basically the same concept. A quick look at the documentation for both and pages like collectionList[0] in TVML and the Getting Started page for UWP[1] look just like shitty HTML.
Ultimately, I can see why they would invent their own UI languages, because it means they don't have an external standard to comply with and can move pretty quickly. I think this is the major reason, not that those other markup languages they created are magically better than HTML's standard. Again, I think this is less of an indictment of HTML than it is companies trying to move as fast as they can without care for interop -- great for them, but probably bad for developers who have to learn all their little languages that are re-creations of an established standard.
[0]: https://developer.apple.com/documentation/tvml/layout_elemen...
[1]: https://docs.microsoft.com/en-us/windows/uwp/get-started/con...