France was an early player in the development of nuclear research, before this was disrupted by their invasion in World War II. In the postwar, the French government supported continued work in the area of nuclear power and — eventually — weapons, but it took some years for this to get back "on its feet." De Gaulle in particular, in the 1950s, saw nuclear weapons as an essential characteristic of maintaining French independence and sovereignty in the face of US dominance of NATO.

What this resulted in was a large commitment to nuclear power reactors as a means initially of generating plutonium, but also as a prestige technology that would allow France to maintain something of a great-power status even though it was getting rapidly eclipsed by the USA and USSR (and losing its colonial holdings, etc.). In the 1970s this morphed into a major commitment to develop civilian nuclear power as both a form of energy independence (this 1973 oil crisis played a role in this) but also as a form of technocracy. The French system of governmental expertise, even more so than the systems of other European nations, concentrates decisions like this in a small body of experts, as opposed to making it up for popular referendum or opinion. (The United States, it should be noted, considers such an approach to be somewhat anti-democratic, which is why we have so much public input, lawsuits, etc., over controversial technologies.)

In 1974 the goal under Prime Minister Messmer (the "Messmer plan") was explicitly to make France as "all nuclear" as possible, the only nation to ever set such an ambitious goal that I have heard of. (It is worth noting that while France has the highest percentage of its power generated by nuclear, it is not the highest-generating nuclear country out there. To my knowledge that status is still held by the USA, which has a relatively small percentage of its energy mix provided by nuclear but still has far more nuclear plants than France does. It just uses way more energy than France.) Again, there is a complex mix of forces here: the fact that nuclear energy siting and work was subsidized if not wholly owned by the state; a connection between nuclear weapons and power infrastructure that was encouraged by the state; the fact that France's system of government expertise was arranged in a way that allowed for this kind of thing to be relatively easily accomplished (in a way that would not have worked in many countries); and the essential argument, made by the French government to its people, that ultimately this would result in cheaper energy costs. Most of the reactors in the current French fleet were built between 1971 and 1982 or so, though there were continued additions through the late 1990s. But after that there was essentially a stall, with only one project that is supposed to go online since 2000. Obviously the Messmer plan did not realize its total ambitions (France is not "all nuclear") but it has come closer than any other nation.

Today it is worth noting that some of these parameters have changed. The economic aspects are not going as smoothly as promised, and French public opinion, in the wake of accidents like Chernobyl and especially Fukushima, has turned remarkably (for the country) anti-nuclear (>50% of those polled basically are in favor of stopping further development or shutting them down).

But again, the TLDR: the French government decided, on the basis of expert advice and prestige, to embark on a radically ambitious nuclear power program in the 1970s, which it saw through the 1990s.

On the history of French nuclear power, see esp. Gabrielle Hecht, The Radiance of France, and also see Christina Stuart, "The French Exception: The French nuclear power industry and its influence on political plans to transition to a new energy system," in the book Learning from Fukushima.