This is a good question because it speaks to the history of human rights, a relatively new historical sub-field. First of all I'd like to address your premise. I wouldn't say that most countries tortured or executed POWs. This was primarily an issue in Germany, USSR, and Japan (not exclusively of course). I won't go into all the reasons why people were tortured and killed because they are numerous and easy to look up. What is more interesting is why the Geneva Convention didn't protect them.

Today, one could argue that human rights has become the primary lens through which Americans (and most other people) perceive the world and their place within it. The concept of human rights, however, is quite recent. In fact, periodization remains one of the central debates within the field. Firstly, what is human rights? The answer seems obvious to anyone alive today but in the 1940s the concept simply didn't exist. Kenneth Cmiel and Samuel Moyn, two early pioneers in the field, defined it as a "thin cultural message" and a universalism respectively. For Cmiel, human rights is a concept that transcends language and international borders. It is the idea that all humans have individual, inherent rights that supersede the laws and actions of the state. An example of what I'm talking about is the famous picture of the man standing down a tank in Tiananmen Square. Americans who saw that picture didn't need to know anything about Chinese history or the Chinese language, heck they didn't even need to know China existed, to understand the human rights crisis it depicted. Similarly, Moyn described human rights as a universalism, something that transcends international borders and is accepted as the predominant morality (people have to believe in it). Moyn argues that the concept of human rights does not have a deeper chronology rooted in say Christianity or civil rights or Greek philosophy or what have you because of what these sets of beliefs (aka universalisms) achieve. Let's look briefly at civil rights. On the surface they seem indistinguishable from human rights but in fact they are entirely different concepts. Civil rights refer to political rights afforded by a sovereign state. Human rights supersede sovereignty. This is the key difference. All these other universalisms operate within the framework of state sovereignty. As we will see, sovereignty is one of the key issues in the history of human rights.

Now that we know what human rights are, let's get back to your question. By the 1940s the term human rights was virtually unknown aside from some superfluous mentions in newspapers. Even still, the concept was undefined and meant different things to different people if it meant anything at all. As an example, Roosevelt intended his Four Freedoms to be subordinated to the authority of the state whereas Nelson Mandela immediately interpreted them as a concept existing beyond sovereignty. The periodization question revolves around when human rights began to matter. The two decades in question are the 1940s and the 1970s with a few historian arguing for the late 1960s. The current consensus is that it was the 1970s when human rights truly began to matter. But the emergence of human rights in the 1970s would not have been possible without the groundwork laid in the 1940s. For this period see the work of Elizabeth Borgwardt, who makes a very compelling argument that rights talk during WWII and immediately afterward was a "New Deal idiom." It would take another essay to describe this but basically Roosevelt's approach to the new international order was rooted in his depression era experience with New Deal policies. Essentially, Roosevelt and Churchill set the tone for human rights with the Atlantic Charter and subsequent Bretton Woods conference. The internationalist language of the Atlantic Charter resonated with people. During the UN Charter Conference in San Francisco, NGOs and states worked toward securing a new world order corrective for the League of Nations. The LON was impotent as I'm sure you are aware. Even here, however, with the crimes of the holocaust known to the world, human rights was an afterthought. The Universal Declaration of Human Rights (UDHR) was pushed through by NGOs at the last minute with a passionate appeal to Secretary of State Stettinius during a late night hotel meeting (debate regarding the hidden politics of this meeting remains ongoing and is incredibly fascinating. Mainly, Stettinius secretly wanted to include the human rights language to ensure senate ratification of the UN Charter in order to avoid a repetition of the calamitous rejection of the League of Nations Charter).

So while the UDHR made it into the UN Charter, it was still an after thought. Civil rights groups immediately seized upon the language and leveraged it in court cases. The California Alien Land Law was struck down due to a successful appeal to the UDHR because as an international treaty the terms set forth are legally binding to all signatories (Namba v McCourt). Well this caused quite a stir in domestic politics. As mentioned before, this was seen as a grave threat to sovereignty. Senator John Bricker introduced a bill opposing the UDHR and UN Charter following this court ruling. The infamous Bricker amendment never passed but did force Eisenhower to commit to refraining from any future involvement with international human rights treaties. In response, civil rights groups abandoned the language of human rights due to its political volatility.

From the 1950s to the 1970s, human rights remained a dormant concept. The Vietnam War, Amnesty International, the Helsinki accords, Soviet dissidents, Congressional power struggles, mass media, global capitalism, détente, the Greece crisis, Rhodesia, apartheid, South American dictatorships, etc. all contributed to an international sense of human rights. There are a number of movements such as anti-colonialism that would not be counted as human rights movements. For an in depth explanation of this I would refer you to Samuel Moyn's The Last Utopia. Again, the key issue here is the concept of sovereignty. The supranational institutions established in the 1940s (the IMF, World Bank, International Court, etc.) facilitated this transition toward a human rights oriented world view. For a more in depth explanation of these factors see the work of Sarah Snyder and Barbara Keys. My favorite contribution to the field however is Mark Bradley's The World Reimagined. Bradley treats human rights as moral language. It is quotidian and frames our understanding of the world. If there was only one book you were to read on the subject it should be this one.

Frankly, human rights didn't exist prior to the 1970s and in regards to the Geneva Conventions they were virtually unenforceable because 1. enforcement would violate state sovereignty, something deemed more problematic than the torture of prisoners 2. there were no mechanisms established that actually could enforce them. The enforcement mechanisms established under the League of Nations were impotent precisely because they were subordinate to the primacy of state sovereignty. Any action that would violate sovereignty was rejected because of the implications for the stability of the prosecuting states 3. the concept of human rights was as of yet undefined and unknown. People didn't see this torture as a crime against humanity, rather it was just brutality against soldiers which was simply a natural consequence of war. This isn't to say people weren't concerned about it, just that it didn't merit international prosecution. Any attempts at prosecution were expected to be conducted by the offending state. You can see the problem in that I assume.

The problem of sovereignty never really went away. To this day lingering apprehensions over sovereignty diminish the efficacy of human rights enforcement. In addition, and something I haven't really touched upon, what rights are considered human rights (political, economic, cultural, etc) have evolved over time. For instance, rape was not originally a human rights offense and was only added decades after the UDHR was ratified. I didn't speak to this dimension of the history because it wasn't most directly relevant and it would have required a doubly long response.

I tried to curtail the length of this response as best as I could. The history of human rights is an important sub field in my opinion. The study of America in the World often takes for granted that between 1945 and 1990 America influenced the world more than the world influenced America. Human rights turns that narrative on its head. When it comes to human rights America has always been behind the curve (excluding a brief moment in the 40s). Please, if you or anyone have any other questions regarding the history of human rights feel free to ask.