Everyone was born with a certain level of connection with nature, and it had to be at a certain level before they would be able to qualify for beast-taming classes.
Researchers haven't found a reason for this or how to increase one's connection to nature, but they had discovered how to measure the level of connection. After a lot of research was carried out on multiple people, they noticed that those who had awakened the light or plant element always had a high connection with nature.
With this limitation and the fact that most people preferred a direct approach to power, there weren't many beast tamers.
In the case of the Nystrom demigod, he had awakened both the plant and light element, making his connection to nature the highest in the city, thus easily making him the best beast tamer in Calton city.