Because most tall NBA players dominated with their height and/or athletic advantage, and didn’t bother to improve their skills as much as shorter less athletic players.
Thing is when you’re talking about the NBA, you’re talking about an elite group of players right? Now think about players who are 5′11 to 6′3 – these group of players have more competition to get into the NBA because there’s simply more people that exist that are that height.
Thus, if you see a short player in the NBA then it’s highly likely they are very good shooters and very skilled. Because if they weren’t, then they wouldn’t make it into the NBA.
Same for white players. Why do white players tend to shoot better ? Because white players are generally less athletic than black players and have to compensate for their lack of athleticism by being more skilled.
Thus that’s why the white players in general tend to be great shooters, from Steve Nash and Mark Price to Jerry West and John Stockton and Larry Bird and Dirk Nowitkzi. They weren’t athletic so they had to compensate by being highly skilled shooters.
And short guys like Isiah Thomas, Tiny Archibald, CP3, Kevin Johnson, Allen Iverson etc had to be highly skilled with some ability like penetration, ball handling, driving, or have high basketball IQ etc else they wouldn’t be in the NBA over a taller player.
Now for taller players, because they are a smaller segment of the population, they can get into the NBA easier just by virtue of being more rare. And if you have a tall player on the team you would maximize the most damage they would do by putting them near the basket and working on the their post up game. Only recently that started to change and now big guys have to be good shooters as well, but I would say that limits their effectiveness since now you are taking them out of position for rebounding or rim protection.
So in the past you had guys like Dr J, George McGinnis, Artis Gilmore etc who didn’t need to have much skill to dominate because they were super athletic and/or super tall. And even in the modern era with guys like Shaq and Giannis being criticized for relying on their brute strength and athleticism too much.
So in general, shorter players and white players are more skilled because they have to be in order to make the NBA to compete with taller more athletic players, and taller / athletic players are less skilled because they can rely on their height and athleticism.
But this isn’t always the case – sometimes you have players who have everything – height, athleticism AND skill / BBIQ. That’s why guys like Jordan, Lebron, KD, Kareem, Magic, Hakeem, Duncan etc are so rare and once in a generation type talents. Because they are tall AND athletic AND skilled.