After having so much fun with Kotlin, I tried switching to Scala and quickly switched back. Scala seems too complicated, with a steep learning curve. Implicits all over the place made it hard to read code and understand what was going on.
I do like many of the more powerful features that Scala has, but Kotlin strikes a right balance of simplicity and power in my opinion.
As a really big Scala fan, I can say Kotlin is probably the right choice when it comes to doing Android development.
That being said, yes Scala has a steeper learning curve than say, Kotlin or Go. But if you're going to devote the I don't know, next ten years of your life to working in a particular language, does it really matter if it takes two months vs. four months to ramp up?
In the mobile space, I see a lot of "also ran" languages, never mind the platform. I don't think people who wrote iOS apps were much intent on using Objective-C outside of that, and I don't think Swift will improve much on this. A lot of Android developers fall into the same niche: They're using Java because they have to, not because it's their major background. So anything that makes their side errands into Android-land easier is welcome (cf. react-native).
Plenty of "ok-ish, but not too hard to learn" languages proliferate in that kind of climate, like go for small web services (where Erlang was too much of an investment). The Unix approach of 1001 ad hoc sub-languages (awk, pic, ed...) adapted.
Quite different from e.g. a J2EE developer's approach, who tends to think of career-defining technologies and languages, where Scala seems like a wise investment.
I do like many of the more powerful features that Scala has, but Kotlin strikes a right balance of simplicity and power in my opinion.