[Question] Why StringProtocol not inherit RangeReplaceableCollection?

Hi everyone,

I have a question about StringProtocol.
Since String and Substring are adopted RangeReplaceableCollection, why StringProtocol not inherit RangeReplaceableCollection?

Also consider StringProtocol could inherit:
CustomDebugStringConvertible
CustomPlaygroundQuickLookable
CustomReflectable
CustomStringConvertible
CVarArg
Thanks!