I'm really rooting for kotlin - it seems like a good runner up to scala without the poor compile times. Looking forward to a 1.0 release
That said, I never figured out why Xtend (http://www.eclipse.org/xtend/) never caught on, and it delivered most of what Kotlin is now offering, but with an even more Java-like syntax right out-of-the-box. Since Kotlin and Xtend are very, very similar, I suspect that Kotlin's ability to genuinely succeed will be mostly based on JetBrains' popularity how strongly they push it.
> ... with an even more Java-like syntax
I think you have your answer right there.
The other reason is that nobody likes the idea of having a language which compiles to Java source code.
Why not? It seems like a good thing if there is a possibility that you might have to go back to Java (if that make sense) or if some time down the road it is simpler for maintainers to go over to Java by reading the compiler output rather than dealing with xtend, if they don't know it. It could also be great if you want to quickly know what the equivalent of what you are writing in xtend is in java - just compile it. That seems more interactive than having to google or look up on StackOverflow.
Of course this assumes that the compiler output is actually readable.
The only hitch is that the Xtend is still very much a work in progress. It doesn't support inner classes and anonymous classes!
Maybe this will improve with ART.
That being said I agree that jars are bigger and scala generates much more anonymous classes. Even with ProGuard that could be a problem on Android 2.3 or older. And ProGuard is a bit slow (for my game it was taking 30+ seconds for every build). Also Typesafe doesn't have plans for making scala first class citizen of Android.
So while Android development with scala is certainly possible there are few inconveniences which one should be aware of.
Which raises an interesting point: what to people on the outside looks like a paradoxical combination of insane hubris and deep technical conservatism is probably simply what to Google's eyes looks like steady-state, responsible technical problem-solving -- c.f., hhvm and Facebook for a similar approach.
And Kotlin's is "compile faster than Scala"!
For those that don't know this feature is called structural typing: http://en.wikipedia.org/wiki/Structural_type_system
Also, OCaml uses structural typing too.
The fact that mainstream developers don't know it, does not make it something that Go designers invented.