Short answers, pulled from the story.
The Hundred Years' War ended in 1453, leaving a scarred but unified France. National feelings emerged from the conflict to transform the nation from a feudal monarchy into a unitary state.
Unitary systems operate differently because they lack such constitutional protections for regions found in federal states. The central government retains the ability to alter that statute at will and can override decisions made by devolved governments whenever it chooses.
A large majority of these nations, specifically 166 out of 193, utilize a unitary system. This statistical dominance suggests the unitary structure remains the most common form of governance today.
Several former unitary states transitioned into federal systems between 1822 and 2005 including Brazil, Czechoslovakia, Yugoslavia, Tibet, Iraq, Lebanon, and Somalia. These transitions document the fluid nature of sovereign state structures over centuries.