The answer, as I understand, is that .NET has to copy code for each non-reference-type instantiation. .NET gets away with it because most types are reference typed and can use the same code.

In F#, you can use hat types via inlining to get even more power. Like creating a map function that works directly on List, Array, and Set - but without using any common interface. Pretty neat, but it emits the function's IL into every callsite so it can get out of hand.

I recall there being a mailing list thread on Go where the designers addressed this. I've no idea how Go is implemented so perhaps the multiple-copies problem is actually significant.