Well, it's kind of the point. Kotlin is supposed to be light weight and easy. Besides, using Scala for Android, while possible, is cumbersome. Kotlin wins this one hands down.
I tried to use Scala for Android in the past and I gave up after attempting to set it up for a few hours. Admittedly it was a while back, perhaps things have improved.
Your claim that Scala on Android works fine got me interested again, so I googled around, found that blog post from May this year (that's more recent than my own experiments), and apparently there are still hurdles: http://blog.scalac.io/2016/05/19/reflections-on-starting-and...
With this type of issues the thing that I'm wary about is not getting a "hello world" app to compile once, but that I can get a project up and running, then need to add something else to the equation (say, a DI, or some code processor), and things fall apart again in an X-Files fashion.
I've heard that Scala has long build times, too, but maybe it's a thing of the past as well - no first-hand experience here as I never got to that stage.
Just use sbt-android and skip all the other nonsense.
Compilation is impressively fast, as the plugin uses the information from incremental compilation to skip ProGuarding completely for things that haven't changed.