Except Scala brought along some improvements and a whole managerie of other features to manage, Groovy with its gradual typing was simultaneously too far away from Java and yet too similar, and C# means opting into a whole different platform and perpetually living downstream of whatever MS wants to do.

Kotlin absolutely nailed the "meaningfully better than Java while staying pedantically true to the existing semantics" in a way that none of the alternatives did even if they did come first.