Kotlin is getting Google's blessing. Mentioned in Google's Android development docs and tutorials. That's the magic sauce. Since Kotlin is veeery close to Java, it was already working well on Android. It targets the JVM of Java6, so if you compile your Kotlin android app, it looks like a regular thing compiled from Android Studio. Regular bytecode, that then gets recompiled by dx (dalvik compiler into .dex bytecode - see also https://softwareengineering.stackexchange.com/questions/2852... )
And yes, adding new things to Android is ... problematic, because Google doesn't accept pull requests. (It accepts bug reports, and sometimes patches of bugs.) And of course you can propose a big patch adding [see https://source.android.com/source/life-of-a-patch ] .. let's say Swift to the Android Platform, but the project owners are Google employees, and the community cannot really influence what gets accepted. [ https://source.android.com/source/roles ]
Also, it's not hard to package all the stuff a language needs and put it into an APK. It's just messy and there are a lot of "edge cases" and things to work out (like the problem of interacting with the Platform, you'd need to call into a "Java" interface from your language, you'd need to run your app inside ART/Dalvik, maybe fork - https://stackoverflow.com/questions/39006044/repeatedly-fork... ).
And so if you spend half a year just getting something to run, and on the next Android version Google makes a change, and bamm, you might get left in the cold, because security or whatever.
And yes, adding new things to Android is ... problematic, because Google doesn't accept pull requests. (It accepts bug reports, and sometimes patches of bugs.) And of course you can propose a big patch adding [see https://source.android.com/source/life-of-a-patch ] .. let's say Swift to the Android Platform, but the project owners are Google employees, and the community cannot really influence what gets accepted. [ https://source.android.com/source/roles ]