[nostrademons]

That's because the concept of a typed lambda function doesn't really exist in Java; in Java 8 lambdas are syntactic sugar for SAM-type object creation. (There are other interface warts too: @DslMarker and parameterized receivers, as you mention; treatment of void vs. Unit; different default collection types; destructuring data types, and the need to clutter your code with 'new' if you use them a lot; etc.)

It's similar to any interop problem: if you want your library to be accessible from lots of languages, its interface better be the lowest common denominator of the features they support.

There's nothing stopping you, though, from defining your interfaces in Java and then implementing those interfaces in Kotlin. The Kotlin interop code even will do a good job letting you use Kotlin's syntactic sugar for this, eg. you can pass Kotlin lambdas to functions that expect a SAM-type, or use 'with()' to treat a conventional builder pattern as a DSL.