I believe one of the main reasons that F# hasn't never really taken off is that Microsoft isn't afraid to borrow the good parts of F# to C#. (They really should've ported discriminated unions though)

Currently DUs are slated for the next version of c# releasing end of this year. However last I knew they only come boxed which at least to me partly defeats the point of having them (being able to have multiple types inline because of the way they share memory and only have a single size based on compiler optimizations).