You started it. You asked for a reason - I gave you a reason.
Technical reason would be much better :-) I have taken a look at combo box in wxWidgets - most public methods there are virtual.

Funny that you mention it, as Python has the concept of private class components as well.
AFAIK only name mangling - and even then one can reach those members outside the class. It is not used as a mean of hiding data from the user, but to prevent name clashes when it is inherited. When you want to make something private, you just say that it is private. If one uses it then, then it's his fault if his software gets broken.