Because France and Germany are about to have major elections. The heads of Government are the most important institution in the European Union, not Junker or any of the so called president offices within the EU. They are powerless, always have been always will be. It is outright stupid to start negotiations when you can't be sure that those who you are talking to now will be the same ones in a years time when the talks heat up. It's best to wait and see on French and German elections and then start since you'll be sure of who the hell you're talking to.