To answer your three questions (or was it two questions?):-
1) Because he doesn't need to.
2) Try.
3) I can't see what you're asking with your final question-mark.
BTW, Is there a "principle of non-contradiction"? Churchmen and women (including many of our saints) have been contradicting each other, on numerous points of doctrine and practice, for the past 2000 years. I guess it will be ever thus. 
Interpretations develop. No Papal document will ever say that a previous document was wrong or in any way deficient. It's not necessary for this to be stated. More important is to claim continuity and consistency. This is how the Church has always worked. Not ideal - but I'll accept it as necessary And so must you. 
It is what it is.