Because politics, Google wanted to do an hostile takeover JavaScript, it backfired, Chrome dropped DartiumVM, and they only got rescued thanks AdWords.

Key language designers left after this.

However it seems to have enough management support, that since it found an home in Flutter, I would assert there are having more adoption than Xamarin, React Native or Cordova.

Dart is advertised as a client-optimized language. Optimized for UI is what the home page says. Say Goodbye to convincing people to use it on the server.