Here's my reason, rooted in practicality and not taste: For me, an ideal app framework would cover both web and native apps. All the major native app platforms rely at least partly on AOT compilation. Android, in particular, is based on a runtime that resembles the JVM at the API level and takes JVM bytecode as input at build time. So for me, the ideal app framework would be in a language that's amenable to compiling ahead of time to efficient JVM bytecode, as well as efficient JavaScript and, for good measure, efficient native code for iOS and Windows.

I think Kotlin is becoming a good choice for these criteria. If you want to follow Mr. Harris's framework as compiler approach, Scala may also be a good choice, because of all the compile-time metaprogramming facilities it has. I don't know how Scala Native stacks up beside Kotlin Native, though, and I've read that Scala 2.12 requires Java 8, so that's probably not a good choice for downlevel Android support.