Because diplomacy in PON was (1) one of the focus of the game, contrary to more wargame-oriented games likes TYW and because (2) it is an extremely difficult thing to set up right.
The good news is that diplomacy in WON derives from PON, only with more and better things
As the French, if you declare war on say Prussia, seeing all your satellites and most of your minor allies do the same is a nice thing to view!