Because that was the philosophy from the beginning ... don't change the language until it's too late, make sure to cripple all new features to maintain backwards compatibility, leave the problems to the tool-vendors to solve.

Actually this philosophy worked better for future-proofing the platform in a weird way. God knows how they would've crippled the JVM if their generics support wasn't made through type-erasure for example.

And so the JVM bytecode is pretty light, now only if they added tail-calls and continuations. But I don't think that will happen too soon.

I understand the philosophy and yes that has in a way future-proofed the JVM. The question is "Does it make sense to try and include advanced language features in a round-about manner into Java ?" Surely this approach only leads to additional complexity and broken features.

Why not put that effort into a newer language running on the JVM ?