Not Android: they run their own OS, they've been around since forever (they predate both Chromecasts and Android TV), and according to Wikipedia they are the "leading streaming TV platform in the US in terms of market share. The Roku OS also holds a noticeable 30.5% share of the global TV streaming hours".

For a long, long time they used to be the "neutral" choice: while Amazon and Google were squabbling and removing each other's apps/products, Roku didn't have any beef with anybody and would therefore have the most complete selection of available streaming apps.