For example, in France - you go from "Clovis, King of the Francs" to "Louis XIV, King of France". At what point did the nomenclature change to be based on the land that the monarch 'owned' (de jure or de facto), compared to the people who inhabited it?
I don't have an answer as to "why", but I have something as to "when", at least for France.
It was Philip II "Augustus" who was the first monarch to use the title Rex Franciae (King of France/Francia) instead of Rex Francorum (King of the Franks), altough the treaties he signed all use the title of "King of the Franks", which tends to show that the change was gradual. The first documents to indicate a shift are royal acts of 1190 employing the term Philippus Rex Franciae, something Bernard Guenée links to Plantagenêt influence. Indeed, at the time kings of England were already using the title Rex Angliae (King of England). Compare this to a contemporary, Henry VI of the Roman Empire, who was Rex Romanorum (King of the Romans). In 1204, the royal protocol uses for the first time Philippus, Rex Francie, and the name of the kingdom, Regnum Francie, appears in 1205. The debate surrounds Latin nomenclature, as as far as I know the title in French always was Roi de France, save for the circumstances I'm describing below.
Your question is interesting because it assumes that the shift was in one direction to another. There was an intense debate during and following the French Revolution, esp. when a Constitutional monarch was considered. Louis XVI, between 1791 and 1792, was indeed Roi des Français instead of his former title of Roi de France et de Navarre. This is a consequence of newfound ideals of popular sovereignty, where the People and no longer God grant the monarch his legitimacy. The only other Roi des Français was Louis-Philippe I, who reigned during the July Monarchy (1830-1848). The title was officially proclaimed the day he was crowned, and it was clearly the result of political compromise, as Louis-Philippe intended to create a junction between revolutionary ideals and the tradition of French monarchy. A similar debate surrounded the possible appointment of Henri d'Artois (or Henri de France) as the monarchists had gained a majority in the Assembly in snap elections following the collapse of the Second Empire (in 1871). However, Henri didn't become king, for a very interesting reason : he refused the Tricolore flag, arguing that he couldn't become king under a flag that symbolized the slaughter of his ancestors, and asked for the bourbon white flag to be reinstored instead. This didn't do, and he was the very last serious pretender to the French throne.