The Java implementation required modifying the IO routines, and this is greatly helped by Java interop being far from easy. .NET was always more friendly to int

The Java implementation required modifying the IO routines, and this is greatly helped by Java interop being far from easy. .NET was always more friendly to interop, lots of projects would be affected. Fixing this would either be a serious break in the ecosystem, or you'd have a new capacity with way too many asterisks to be useful. The .NET world already spent the budget for big migrations with .NET Framework -> Core (just like Java did with Java 8->9, or Python with 2->3) - as much as we might like green threads, they aren't useful enough (compared to async/await) to justify another break.