If I remember correctly the CLR implementation of Scala was scrapped because there were some limitations with the CLR's reified generic type system that made it impossible to express scala's type system.

I even believe that Microsoft research tried to find a solution for a while, but were unable. I'll try and find some references if I can.

Edit:

This link has some good info in it: http://visualstudio.uservoice.com/forums/121579-visual-studi...

Looks like the CLR can't represent higher kinded types at runtime using the reified type system.