The short answer is that the Flutter team wanted a language that was something like Dart, and the Dart team needed a killer app and was willing to put a lot of effort into satisfying Flutter's requirements.
It's unlikely that Flutter could have gotten that level of cooperation and influence over a more successful language, back when they were getting started and nobody had heard of them yet.
It's unlikely that Flutter could have gotten that level of cooperation and influence over a more successful language, back when they were getting started and nobody had heard of them yet.