Kotlin is a very nice language. It has a very good syntax. It is easy to write Kotlin code.
I have found that Dart's syntax is much more akin to C or C++. Kotlin's syntax is much more akin to Java or to iOS's Swift.
As I understand it, Dart was chosen because it would work better with Flutter.
I believe that the issue is this: does there exist a programming language which has a Kotlin-like syntax and which is as optimal for Flutter as Dart?
I believe that the exploration of that question would make for an interesting Medium article.