The JVM doesn't directly support higher-kinded types either. It might be a pain to come up with a scheme that interops nicely with other .NET languages, but in principle there doesn't seem to be anything that would make it impossible to build a Scala compiler for .NET (though it might involve lots of boxing and downcasts).

It kind of worked (I actually used it), but I don't think there was enough interest in it. Java is clearly dated on the JVM, but C# isn't that bad, and there is also F#. The CLR just doesn't need Scala as bad as the JVM did (and vice versa). There was never going to be a strong ecosystem there, even if there was a perfect version of scalac working.

You can easily erase to the CLR like you can on the JVM, the only issue is interop with .net APIs that use generics, of which this mostly means LINQ.