It's not .NET vs Java so much that the development of the two languages you mention are coordinated. For Kotlin and Clojure interop works both ways; Scala doesn't care much about that, so Scala developers write special facades as Java language API. There are lots of different languages on Java, and they care about different things. The Java team itself develops only one language, although at one time there was another (JavaFX Script). Some language developers (Kotlin, JRuby) work more closely with the Java team, and others (Scala) hardly ever do.