The interface of an object is the part of that object that is visible to external users of that class.
On the contrary, protected and private methods (and fields) belong to the class internals. They are encapsulated inside the class and a class user should not be aware of them.
So, since interface is used to define interfaces (no pun intended), it is reasonable that they do not contain protected methods.
One doesn't want to think of implementation when defining an interface
