Encapsulation is one of the main features of OO. Encapsulating your data in classes means that users of the class can not break the class' data's invariants, because the class' state can only be manipulated through its member functions.
If you allow derived classes access to their base class' data, then derived classes need to take care to not to invalidate the base class' data's invariants. That throws encapsulation out of the window and is just wrong. (So do getters and setters, BTW.)
I find myself using protected less and less over the years, even for member functions. If a class fully implements a simple concept, then all of its state should be manipulatable through its public interface. If derived classes need "back doors" to sneak in, then I usually question my design. (Which isn't to say I never use protected. I just find I need it less and less.)
