The truth of it is that Kotlin is incredibly easy to learn, even for non-Java devs. It is definitely focused on a very low barrier to entry. I don't think this was an original goal but it's become a significant focus of the language especially after being selected to be the next platform language for Android.
This isn't true of Scala at all. Beyond things like SBT, comparatively poor IDE support (less of an issue these days) it is also just a very complex language, with many complicated semantics in it's standard library and even more complex/fancy 3rd party libraries piled on top of that.
Scala became an incredibly powerful language, not a simple or easy to use one however.
Kotlin is a very user friendly language that is easy to learn but still has enough power to keep most expressiveness focussed devs happy.
Beyond the learning curve Kotlin integrates with Java a lot better, calling in both directions.
These 2 factors alone probably account for why it was chosen over Scala for Android and in general why it has value over both Java and Scala.
I think with Gradle moving from Groovy -> Kotlin, Android obviously and big companies adopting Kotlin w/Spring on the backend the ship has indeed sailed for Scala, it doesn't really have a place outside of places that specifically want a more FP lang.