Why can't Protocols be nested in other Types?

We can nest classes, structs & enums:

class TestClass {
     class OtherClass {}
     struct OtherStruct {}
     enum OtherEnum {}
}

Why can't we also nest protocols?

class TestClass {
     protocol Delegate {}  // error: Declaration is only valid at file scope
}

So far the workaround is to declare it at file scope using and use a typealias instead.

The underscore prefix makes it clear that the global symbol name is not intended for direct use.

class TestClass {
     typealias Delegate = _TestDelegate
}

protocol _TestDelegate {}

Is this just not yet implemented or is there a reason for not supporting that at all?