It's primarily a result of land solidification. Many of the old kingly titles came from the times of mobile populations. When the Franks are moving about and taking over new lands, Roi des Francs makes sense as you are primarily the head of a population of people rather than a specific geographic area. Secondly, by laying claim to a land area rather than population, you are also seeking to de-legitimize competing dynasties who are seeking to control a similar area.

This comes to a head with Phillip II because he was the one who essentially created the modern geographic definition of France. Before him the Platagenets had a claim of much of the Atlantic coastal regions of the country and were a competing center around which a French country could form. By taking the title of Roi de France and defeating the Angevin empire, he was staking and solidifying the Capetian claim to being the French monarchy, centered around Isle de France (Paris) rather than Normandy and England as the Platagenets were. He was also asserting the claim of the King of France (Isles de France) over Normandy and other area along the coast which had previously been part of the Frankish empire and had subsequently been taken over by the Normans. This was highly significant as it was the basic impetus in the (re)establishment of 1- a strong central monarchy over France and 2- a claim of geographic continuity in France, something which had been broken by the coming of the Vikings.

Source: Baldwin, The Government of Phillip Augustus: Foundations of French Royal Power in the Middle Ages. The Ebook is pricey at 40 bucks, but its worth it (inherited a copy from my grandparents). It's probably the best book for explaining how France established its surprising level of geographic continuity.