Interesting subject.
The biggest factor, I think, is Geography, the size of the country and its isolation from other states being separated on each side by oceans and bounded by the frozen north and the impenetrable jungles of Central America to the south. Most countries in the rest of the world are bounded by neighbouring states in which they are in competition or alliance. The US was only bounded by the British Canada and Mexico and the weak Spanish colonies in the west. It managed to come to a settlement with Canada and pursue an opportunity for westward expansion by small wars and the purchase of territory fairly quickly. America was left to expand while the rest of the world was consumed by their own wars and rivalries. It had the resources to develop a self-sustaining economy.
Secondly, the founding of the US benefitted from the best of Enlightenment thinking and this informed the framing of the constitution and this has proved to be durable and adaptable to serve the huge scale of the country. While one of its key provisions is to provide freedom of religion, the culture that developed in the US has a strong religious element. It is largely Protestant and informed by the emphasis on community, a civic culture and a work ethic. The political development of the rest of the world was stymied by the legacy of absolute Kings and the religious wars. The US got a democratic Parliament quite early on and from that came legal framework that formed the basis of a capitalist economic system. The US managed to achieve a single currency, free movement of capital and labour over a vast territory. It did this quite early and benefitted from the great economies of scale that came with that.
Thirdly immigration. The US would have remained a small group of rural plantation owners if it were not for mass immigration. With the people came ideas and a vitality to exploit the opportunities of this vast territory. The US was able to take great advantage of the technology of the industrial revolution, especially the railways, to consolidate the unify the country and this led to its emergence as a superpower in the twentieth century.
What a lucky country! Unless, of course, you happen to be a native American Indian or a victim of the plantation economy of the South.
However, it also has some serious problems.
It has constitution issues. A Constitution written in the 1790s was largely concerned with the political problems that were current at that time and did not anticipate a federalised super-state. Its terms are anachronistic and changing it is slow and difficult and important issues are left for the interpretation by the supreme court and the political balance of its members. Few countries agonise about their constitution as much as the US. It goes through periods of huge internal tension between the federal government on the states and also between the state and the individual.
The US attitude towards Firearms is a symptom of this. While people may claim it is about the freedom to play at being a romantic frontiersman hunting and shooting the wildlife. It is not just a rural thing. There is a deeply entrenched suspicion of the government and the federal government in particular. There are plenty of people in the US who really want nothing to do with government and the gun is the symbol of defiant independence and they claim the right to bear arms in the constitution guarantees this.
Other countries do simply do not have this political argument.
Then there is the religiosity of Americans. Many of the immigrants that founded the country were escaping religious persecution and the US was guaranteed freedom of religion. While taking advantage of this freedom, religious groups are very active in promoting their views and politicising issues that they regard as important. Arguments over the content of schoolbooks, creationism and abortion. This holds the US back in important areas like genetic research.
Other countries don’t have these arguments, they don’t have the ‘Religious Right’.
The Cold War? That was the result of WW2 and that had the greatest change in the US. It consolidated its status as a global superpower with a huge industrialised economy largely unscathed by war. But the lesson of WW2 was that the US cannot be confident its geographic isolation is sufficient defence from the other powerful states. It suffered a surprise attack by the Japanese and this abruptly ended US isolationism. By the end of the war, it was clear developments in aircraft and the ballistic missile had changed everything and the US had to engage in world politics. The US had to compete with the USSR and China who had a mission to spread their ideology around the world. This rivalry and the proxy wars the resulted polarised the politics in many countries between left and right. The US retrenched because of anti-communism.
In other democracies, the debate was more nuanced because the benefits of a centrally planned economy were self-evident in wartime. European states were able to decide which parts of the economy should remain state-controlled and which parts should be left to market forces. It was also informed by pre-war thinking on reforms that followed the Great Depression. Most created state provided housing, education, social security and healthcare systems. A lot of this was built from scratch in countries devastated by war. Just as the US benefitted from being able to create a state from a clean slate in the 1790s, western Europe was able to do the same in the 1940s. They too were able to create constitutions that seek to protect them against the political disasters they had recently experienced.
These debates did not seem to happen in the US and it was left with a private insurance-based healthcare system which is hugely expensive and not at all comprehensive and very patchy welfare provision and social security and an expensive private financed education system. These are major differences with other states, while some of the reasons may have been ideological, the wealth of the country after WW2 would have played a part because these issues may not have risen up the political agenda.
The way to win votes in the US today is to promise a return to that great post-war age of prosperity.