The number of players was already mentioned. But lets go one level deeper than that.
There are two driving factors in the chance that a player becomes a top player:
- Talent
- Education
And once you know the chance that someone becomes a top player, you can multiply this with the number of players. Which depends on the population and chance of someone playing.
So aA very simplistic formula wouldmight be:
Expected number of top players = Population size * likilyhoodlikelihood to play
- likelihood to have enough chess talent * likelihood to have enough chess education
Now, let's look into each of the factors:
#Likelihood to have enough chess education
This is both cultural and economical.
Beginner level: The more people play chess, the more likely you have access to someone who can explain you the rules, and teach you how to play
Intermediate level: If more people play, you are more likely to find suitable opponents to develop against. If the local economy is better, it is more likely that you can afford basic training materials or have the chance to play/study online. A combination of economy and player density will determine if you are able to play many tournaments.
Advanced level: At this point training will require a serious investment of time and money that not everyone can make. Logistics are slightly easier if there are several strong tournaments nearby, but you will still expect to travel international. Also don't underestimate the fact that you will likely not be able to work (or at least less) outside chess if you want to climb to the top.
#Likelihood to play
Once a sport is popular, it is much more likely to be picked up by new people. A clear example can be found in the Netherlands, where the sport got really booming for decades after Max Euwe became the world champion. Not only will this lead to more players, it will often also lead to more facilitating infrastructure/clubs which may encourage people to play.
#Likelihood to have enough chess talent
There are plenty of controversial statements that could be made here, but I think it is fairly safe to say that starting with general education and calculation/reading skills have a positive impact on chess talent coming out.
#Population size
More people leads to more potential players.
#Conclusion
The reason why there are less Black top players, is because the population in general has a lower average likelihood to become one. For people in less faverable circumstances the answer so far should already explain things clearly. For other people (e.g. those in the US) I think the main reason is that they have a reduced likelihood to become a player. If earlier generations did not play themselves, and there are no past champions to inspire you, that will slow down adoption.
That being said, as education and economical development increases across the globe, I think we just need one black world champion of a few top players in general before we start seeing many black players grow to the top :)