A much, much better question is what united France (and revealingly, how it was united) during this period. Once you answer that, you answer your first question.
The seizure of Normandy and the lands north of the Loire under Philip II Augustus greatly expanded the royal demesne and ended the problem that plagued earlier French rulers of not having any say outside the Ile de France, followed by Louis VIII and St. Louis expanding French power into the Languedoc through the Albigensian Crusade, thus leaving a large and powerful swathe of land under Capetian hands, compared to the utter collapse of the Hohenstaufen Kingdom of Germany (and Sicily) within the HRE and the position where the Emperor had less real power than many of his vassals?
Was thinking of making an effortpost, but...this will do for now.
