If Flutter works for you, keep using it. It is open source (!= Google service), it is productive, and there are no signs that Google is abandoning it; on the contrary, it seems that more efforts are put behind: Dart language and tooling improved a lot since 2.0 (+ non-nullable types coming soon), Flutter getting support for Web, Mac and potentially other platforms...
1. It was released before Flutter, 2015 compared to Flutter's 2017
2. No need to learn a new language. JavaScript is JavaScript. The initial thought of having to learn Dart (which is very similar to JavaScript and Java, but who'll know till they use it?) is a push back
3. RN's similarity to React. It is easier for devs who'd been using React to jump into it, and React has more adoption than Angular
In the end, users don't really care whatever you use. Does your app do what they want/expect?