Haskell/ML families bear only the slightest similarity to C#. Saying 'Haskell has a REPL, so why doesn't C#' is like saying 'C and GLSL both have structs, so why doesn't C run on my GPU?'*

Furthermore, like I said in another comment, a C# REPL in that era would have been nearly unusable. The language is not designed for incremental authoring or compilation.

* CUDA notwithstanding