I believe Flutter was chosen because of somewhat easy way of keeping common codebase for both iOS and Android clients. Not trivial, and at least it renders natively :V
Flutter "compiles to native," but the UI is just a giant canvas they paint themselves. React Native uses real native views, so you get actual platform widgets, accessibility, and OS-level optimizations instead of shipping your own game engine.
Also, Google has a habit of hyping projects then quietly killing them (I sadly took the Polymer ride).
