This raises two questions:
1) Why is this the case?
2) What exactly are hazel eyes?
Hazel eyes are eyes that have a combination of green, gold, and brown coloring, which sets them apart from most other eyes, which are a solid color. The amount of each color can vary among different people with hazel eyes, which can cause hazel green eyes or hazel brown eyes.
How common are hazel eyes?
Hazel eyes are fairly rare, and only about 5% of the population has them.
Hazel is not an eye color but rather a condition where several eye colors are present.
Now the question is why.
This is an area which needs to be explored further. Similar to the high frequencies of red and reddish hair among Rh(D) negative individuals.
What is the reason?
While usually people have brown or blue eyes, depending on which of the 2 genes from each parent gets inherited and which one is dominant, but in Rh(D) negative individuals, there doesn’t seem to be the pure dominance of one trait over the other. Much rather, Rh(D) negative individuals tend to show traits from various parts of ancestries within our phenotype. This might take us back to the pre-Yamnaya Black Sea region where around 7,000 ybp the first blue eye mutation might have taken place and where red hair in humans first came about. Is it possible that the first gene deletion responsible for so many Europeans being Rh(D) negative too place there as well?
Absolutely!
Prior to that, all humans had brown eyes, so the early mixing with those carrying the new mutation may have led to hazel eyes early. While red hair is recessive, it also may have started to show up in dominant phenotypes.
This is not something easy to prove and because of that, various studies showing frequency differences based on Rh factor are not yet published.
In most of Africa and Asia, Rh(D) negatives are actually weak D. It is possible that the gene deletion leading to weak D or complete deletion of the D has taken place in different parts of the world at different times. This is something that hasn’t been proposed before, but it is highly plausible for the same evolutionary step to take place in more than one region.
If all three mutations, blue eyes, red hair and Rh(D) gene deletion took place within the same region, this might be one possibility as to why despite the dominant brown eyes being passed on, the blue still shines through to create a hazel, the red still shimmers through in those having one of the other 3 hair colors.