Why are there so few minority designers…?

What is a minority designer?

The answer is in the question itself. There are simply fewer minorities in everything, by definition.

designing is not considered cool, within some cultures, it is not a race thing, it is a cultural thing.

Perhaps they already represent an amount in the discipline that is proportional to their population.

There are many minorities designers, just look on Dribbble or Behance. If you are referring to minority designers represented at well-known startups, then, unfortunately, people like to hire other people who are like themselves.