There are better languages with better ecosystems. Even the Android team picked Kotlin. It failed going into the realm of JavaScript (Typescript took that).
It's not that it's a terrible language. It isn't. There are just better languages that don't have the burden of Google who is notoriously flaky on supporting failed projects.
Flutter succeeded because the alternatives are worse and because of China which is its main source of traction.
September 2018 - Flutter release preview 2 released (can't find the date on RP1)
December 2018 - 1.0 release of flutter
Flutter is not going to be the primary framework over JVM-based for Android any time soon, these things takes lots of time. But android didn't pick Kotlin over flutter, it was just not ready to be the preferred language at that point in time. I would even say Kotlin could have been promoted even sooner then it was
It's not that it's a terrible language. It isn't. There are just better languages that don't have the burden of Google who is notoriously flaky on supporting failed projects.
Flutter succeeded because the alternatives are worse and because of China which is its main source of traction.