Performant implementation of discriminated unions in .NET (unlike any that you would see in JVM) requires modification of both the type system and the runtime to account for non-boxing representation of struct unions as well as unions in which GC references alias value types/structs. This is highly non-trivial and the people who could be working on this are currently busy with other features namely Rust-like traits called explicit and implicit extensions. Don't forget that .NET is worked on by at least 10x smaller community and teams than JVM implementations and yet delivers much better YoY degree of improvement and much nicer user experience (Gradle lol).

In addition, green threads approach is a strictly worse and a more limited solution that only solves thread blocking without unlocking the variety of terse concurrency patterns enabled by the way the tasks are handled in .NET. They also add significant overhead to interop with native code due to thread pinning/stack switching (also applies to Go).