C# :: Why Can't A Constant Field Be Of Non-built-in Struct Type

Code:

class T
{
enum E { }
struct S { }
interface I { }
delegate void D();
class C { }
}

[code]....

All of the above are possible except the constant field of a type that is a struct. I can see why the non-string reference types would default to the only literal expression they can represent - null - and if I'm not mistaken the default value of a struct is an instance of the struct with all its fields set to their default value, but why is it that a const struct field is not possible? is there a reason why or is it just the way c# was written? oh and by the way, what's the idea of allowing enum types to have a default constructor?