Throughout the 1940s and 1950s, Superman was the most popular character in the comic books. The first Superman movie was released in the USA in 1951. From 1951 to 1991, 5 Superman movies were released globally. These Hollywood Superman movies got huge success globally. The theme of the movies were truth, justice and the American way of life. i.e capitalism. One of the key factors of the success of these movies was their script.
The second key factor was Hollywood technology, which was upgraded continuously. We cannot deny these factors in the popularity of these movies. But there is also one factor which is grossly ignored by us while evaluating the success of Hollywood movies. That factor is a psychological factor.
As we know that five Superman movies were released between 1951 to 1991, this period was the period of the cold war era. This cold war was fought on the ideology of capitalism and communism. America was representative of capitalism while the USSR was representing communism. Thus the cold war was the rivalry between the USA and USSR.
During this period, there was continuous fear of nuclear war between the USA and the USSR. We have the incidence of the Cuban crisis of 1962. The USSR had installed nuclear warheads in Cuba and threatened the near existence of the USA. Thus not only the USA people but the global population was under the psychological fear of nuclear war. There was always a fear of death or nuclear catastrophe. So, the people were searching for someone who could save them if something unpleasant happened.
The people got the protector or saviour in the form of Superman in Hollywood movies. As the name suggests, Superman had qualities which were seldomly possessed by the common-man. In most of the Superman movies, he had been shown as the saviour of the world from the enemy. So, the people had connected to Superman psychologically and emotionally. They were looking to Superman as their saviour or protector.
To conclude, the psychological and emotional attachment of people with the character of Superman increases the popularity of Superman movies.