There were Africans running in Ryun's time. Check results from the Mexico City Olympics or ask Ron CLarke about how lucky he was to not have to compete against Africans. But yes, there were fewer Africans running then than now though there were far more than most people today recognize.
There was also a LOT MORE competition from Europe then than there is now and in the non-English world, a lot of those competitors were really professionals. Every world class athlete from the Eastern Bloc "worked" as either a military officer or as a student who received a "stipend." In ether case, the athletes' real job was to train and compete.
Even in non-Eastern Bloc countries, there frequently were support systems that allowed athletes of international caliber to train and race as they needed to without the imposition of work. Kenyans who showed running ability were given jobs in the military or police force where they had very light work loads and were free to travel to races whenever the Kenyan AAA approved a trip. Viren's work as a policeman was essentially part time. In other parts of Europe, athletics was a very popular sport and local clubs were often able to find jobs for their best guys that paid full salaries for part time work that allowed them to train and occasionally that even happened in English speaking countries. Gordon Pirie had a job with a bank where his hours were reduced so he could train. Snell was doing promotional work for a New Zealand cigarette company which didn't take up much time at all when Ryun beat him in 1965. It was a job that was specifically created to allow him to train and race.
If you look at the World Cross Country Championships today or look at results from national championships in Europe, Asia, New Zealand, Mexico, South America, it's easy to claim that "most of the world can't be bothered with competing." There are probably fewer countries today where athletics is taken seriously by far than there was in Ryun's time and many of the athletes he was beating then were mature professionals. And this is why those of us who were involved with the sport in Ryun's time think he was so great.