First, the medical level has improved a lot. Coupled with decades of accumulation, training methods and training supporting equipment have become much more powerful. Even on-the-spot command has undergone great changes due to the improvement of technology and sports intelligence, sports statistics and physiological analysis. For example, the simplest shooting training cannot be improved by simply relying on a large amount of training. Without scientific methods and medical means, your hands or other body parts may be destroyed during the training process, and you can shoot quickly. Shooting with the ball and shooting with the ball both require muscles and coordination. These two types of training also require relatively high sports technology.
Second, fouls are now completely understood by oneself. In fact, in the 1990s, the "theoretical" intensity of physical contact was not much better than it is today. But compared with today, the NBA in the 1990s was particularly prone to problems, because today's rules are all set for you as long as you have had trouble before, and you are no longer allowed to interpret it yourself, but are told how to call it. For the referee, the difficulty suddenly went down.
Thirdly, the regulations in the past were not detailed, and you could challenge the referee's authority on the field, but now players have no right to challenge the referee in the rules. If you argue with the referee, the referee can choose to ignore it. To put it bluntly in that era, referees had nothing to rely on, and players in physical contact could not control the situation in both directions. Now the game is purer in a certain sense, that is, who is the physically/technically better player. There are still all kinds of dirty things beyond the meaning of basketball, but they are much less.
Fourth, today’s players take fewer detours. As long as your physical talent is up to standard, you are young, and you are willing to practice without any technical difficulties, you can reach a level similar to that of Jordan, Chamberlain and others. In fact, the old players are equivalent to bringing out a system for you to practice by yourself, or You can imitate it directly, what needs to be done to pave the way. Finding the right person means finding the right method that belongs to you, and there is no need to make detours at all. In contrast, the old players in the past basically figured it out by themselves through game after game.