Most smart TV platforms that aren't Android TV or Tizen explicitly are HTML5 based, in that their UI is HTML5 based. But smart TV platforms are never write once run anywhere - remote control button patterns, OS weirdness and shoestring amounts of RAM mean you've got to do whole suites of testing and bespoke code for at best every manufacturer, and in practice every set.

LG is the only one that could be said to have sufficient marketshare to justify that investment for any but the largest VOD providers now. The revenue generated from those platforms once rights holders are paid is a fraction of what it costs to maintain them simply through testing overhead. Android TV and Tizen's marketshare has increased rapidly over the last few (read three-ish) years, and when you often need to account for international rollout or repurposing in your cost analysis it's even stronger (Roku has zero marketshare outside of the US, many large cable companies outside of the US have Android TV based set-top boxes with multiple million installations).

This generally also applies to browser based streaming, with the added bonus that is where a bunch of piracy comes from.